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Overview of axisartist toolkit

The axisartist toolkit tutorial.

::: danger Warning

axisartist uses a custom Axes class (derived from the mpl’s original Axes class). As a side effect, some commands (mostly tick-related) do not work.

:::

The axisartist contains a custom Axes class that is meant to support curvilinear grids (e.g., the world coordinate system in astronomy). Unlike mpl’s original Axes class which uses Axes.xaxis and Axes.yaxis to draw ticks, ticklines, etc., axisartist uses a special artist (AxisArtist) that can handle ticks, ticklines, etc. for curved coordinate systems.

Demo Floating Axis

Since it uses special artists, some Matplotlib commands that work on Axes.xaxis and Axes.yaxis may not work.

axisartist

The axisartist module provides a custom (and very experimental) Axes class, where each axis (left, right, top, and bottom) have a separate associated artist which is responsible for drawing the axis-line, ticks, ticklabels, and labels. You can also create your own axis, which can pass through a fixed position in the axes coordinate, or a fixed position in the data coordinate (i.e., the axis floats around when viewlimit changes).

The axes class, by default, has its xaxis and yaxis invisible, and has 4 additional artists which are responsible for drawing the 4 axis spines in “left”, “right”, “bottom”, and “top”. They are accessed as ax.axis[“left”], ax.axis[“right”], and so on, i.e., ax.axis is a dictionary that contains artists (note that ax.axis is still a callable method and it behaves as an original Axes.axis method in Matplotlib).

To create an axes,

  1. import mpl_toolkits.axisartist as AA
  2. fig = plt.figure()
  3. ax = AA.Axes(fig, [0.1, 0.1, 0.8, 0.8])
  4. fig.add_axes(ax)

or to create a subplot

  1. ax = AA.Subplot(fig, 111)
  2. fig.add_subplot(ax)

For example, you can hide the right and top spines using:

  1. ax.axis["right"].set_visible(False)
  2. ax.axis["top"].set_visible(False)

Simple Axisline3

It is also possible to add a horizontal axis. For example, you may have an horizontal axis at y=0 (in data coordinate).

  1. ax.axis["y=0"] = ax.new_floating_axis(nth_coord=0, value=0)

Simple Axisartist1

Or a fixed axis with some offset

  1. # make new (right-side) yaxis, but with some offset
  2. ax.axis["right2"] = ax.new_fixed_axis(loc="right",
  3.               offset=(20, 0))

axisartist with ParasiteAxes

Most commands in the axes_grid1 toolkit can take an axes_class keyword argument, and the commands create an axes of the given class. For example, to create a host subplot with axisartist.Axes,

  1. import mpl_toolkits.axisartist as AA
  2. from mpl_toolkits.axes_grid1 import host_subplot
  4. host = host_subplot(111, axes_class=AA.Axes)

Here is an example that uses ParasiteAxes.

Demo Parasite Axes2

Curvilinear Grid

The motivation behind the AxisArtist module is to support a curvilinear grid and ticks.

Demo Curvelinear Grid

Floating Axes

AxisArtist also supports a Floating Axes whose outer axes are defined as floating axis.

Demo Floating Axes

axisartist namespace

The axisartist namespace includes a derived Axes implementation. The biggest difference is that the artists responsible to draw axis line, ticks, ticklabel and axis labels are separated out from the mpl’s Axis class, which are much more than artists in the original mpl. This change was strongly motivated to support curvilinear grid. Here are a few things that mpl_toolkits.axisartist.Axes is different from original Axes from mpl.

  • Axis elements (axis line(spine), ticks, ticklabel and axis labels) are drawn by a AxisArtist instance. Unlike Axis, left, right, top and bottom axis are drawn by separate artists. And each of them may have different tick location and different tick labels.
  • gridlines are drawn by a Gridlines instance. The change was motivated that in curvilinear coordinate, a gridline may not cross axis-lines (i.e., no associated ticks). In the original Axes class, gridlines are tied to ticks.
  • ticklines can be rotated if necessary (i.e, along the gridlines)

In summary, all these changes was to support

  • a curvilinear grid.
  • a floating axis

Demo Floating Axis

mpl_toolkits.axisartist.Axes class defines a axis attribute, which is a dictionary of AxisArtist instances. By default, the dictionary has 4 AxisArtist instances, responsible for drawing of left, right, bottom and top axis.

xaxis and yaxis attributes are still available, however they are set to not visible. As separate artists are used for rendering axis, some axis-related method in mpl may have no effect. In addition to AxisArtist instances, the mpl_toolkits.axisartist.Axes will have gridlines attribute (Gridlines), which obviously draws grid lines.

In both AxisArtist and Gridlines, the calculation of tick and grid location is delegated to an instance of GridHelper class. mpl_toolkits.axisartist.Axes class uses GridHelperRectlinear as a grid helper. The GridHelperRectlinear class is a wrapper around the xaxis and yaxis of mpl’s original Axes, and it was meant to work as the way how mpl’s original axes works. For example, tick location changes using set_ticks method and etc. should work as expected. But change in artist properties (e.g., color) will not work in general, although some effort has been made so that some often-change attributes (color, etc.) are respected.

AxisArtist

AxisArtist can be considered as a container artist with following attributes which will draw ticks, labels, etc.

  • line
  • major_ticks, major_ticklabels
  • minor_ticks, minor_ticklabels
  • offsetText
  • label

line

Derived from Line2d class. Responsible for drawing a spinal(?) line.

major_ticks, minor_ticks

Derived from Line2d class. Note that ticks are markers.

major_ticklabels, minor_ticklabels

Derived from Text. Note that it is not a list of Text artist, but a single artist (similar to a collection).

axislabel

Derived from Text.

Default AxisArtists

By default, following for axis artists are defined.:

  1. ax.axis["left"], ax.axis["bottom"], ax.axis["right"], ax.axis["top"]

The ticklabels and axislabel of the top and the right axis are set to not visible.

For example, if you want to change the color attributes of major_ticklabels of the bottom x-axis

  1. ax.axis["bottom"].major_ticklabels.set_color("b")

Similarly, to make ticklabels invisible

  1. ax.axis["bottom"].major_ticklabels.set_visible(False)

AxisArtist provides a helper method to control the visibility of ticks, ticklabels, and label. To make ticklabel invisible,

  1. ax.axis["bottom"].toggle(ticklabels=False)

To make all of ticks, ticklabels, and (axis) label invisible

  1. ax.axis["bottom"].toggle(all=False)

To turn all off but ticks on

  1. ax.axis["bottom"].toggle(all=False, ticks=True)

To turn all on but (axis) label off

  1. ax.axis["bottom"].toggle(all=True, label=False))

ax.axis’s getitem method can take multiple axis names. For example, to turn ticklabels of “top” and “right” axis on,

  1. ax.axis["top","right"].toggle(ticklabels=True))

Note that ‘ax.axis[“top”,”right”]’ returns a simple proxy object that translate above code to something like below.

  1. for n in ["top","right"]:
  2.   ax.axis[n].toggle(ticklabels=True))

So, any return values in the for loop are ignored. And you should not use it anything more than a simple method.

Like the list indexing “:” means all items, i.e.,

  1. ax.axis[:].major_ticks.set_color("r")

changes tick color in all axis.

HowTo

  1. Changing tick locations and label.

Same as the original mpl’s axes.:

  1. ax.set_xticks([1,2,3])
  1. Changing axis properties like color, etc.

Change the properties of appropriate artists. For example, to change the color of the ticklabels:

  1. ax.axis["left"].major_ticklabels.set_color("r")

Rotation and Alignment of TickLabels

This is also quite different from the original mpl and can be confusing. When you want to rotate the ticklabels, first consider using “set_axis_direction” method.

  1. ax1.axis["left"].major_ticklabels.set_axis_direction("top")
  2. ax1.axis["right"].label.set_axis_direction("left")

Simple Axis Direction01

The parameter for set_axis_direction is one of [“left”, “right”, “bottom”, “top”].

You must understand some underlying concept of directions.

On the other hand, there is a concept of “axis_direction”. This is a default setting of above properties for each, “bottom”, “left”, “top”, and “right” axis.

? ? left bottom right top

axislabel direction ‘-‘ ‘+’ ‘+’ ‘-‘

axislabel rotation 180 0 0 180

axislabel va center top center bottom

axislabel ha right center right center

ticklabel direction ‘-‘ ‘+’ ‘+’ ‘-‘

ticklabels rotation 90 0 -90 180

ticklabel ha right center right center

ticklabel va center baseline center baseline

And, ‘set_axis_direction(“top”)’ means to adjust the text rotation etc, for settings suitable for “top” axis. The concept of axis direction can be more clear with curved axis.

Demo Axis Direction

The axis_direction can be adjusted in the AxisArtist level, or in the level of its child artists, i.e., ticks, ticklabels, and axis-label.

  1. ax1.axis["left"].set_axis_direction("top")

changes axis_direction of all the associated artist with the “left” axis, while

  1. ax1.axis["left"].major_ticklabels.set_axis_direction("top")

changes the axis_direction of only the major_ticklabels. Note that set_axis_direction in the AxisArtist level changes the ticklabel_direction and label_direction, while changing the axis_direction of ticks, ticklabels, and axis-label does not affect them.

If you want to make ticks outward and ticklabels inside the axes, use invert_ticklabel_direction method.

  1. ax.axis[:].invert_ticklabel_direction()

A related method is “set_tick_out”. It makes ticks outward (as a matter of fact, it makes ticks toward the opposite direction of the default direction).

  1. ax.axis[:].major_ticks.set_tick_out(True)

Simple Axis Direction03

So, in summary,

  • AxisArtist’s methods

set_axis_direction : “left”, “right”, “bottom”, or “top” set_ticklabel_direction : “+” or “-“ set_axislabel_direction : “+” or “-“ invert_ticklabel_direction

  • set_axis_direction : “left”, “right”, “bottom”, or “top”
  • set_ticklabel_direction : “+” or “-“
  • set_axislabel_direction : “+” or “-“
  • invert_ticklabel_direction
  • Ticks’ methods (major_ticks and minor_ticks)

set_tick_out : True or False set_ticksize : size in points

  • set_tick_out : True or False
  • set_ticksize : size in points
  • TickLabels’ methods (major_ticklabels and minor_ticklabels)

set_axis_direction : “left”, “right”, “bottom”, or “top” set_rotation : angle with respect to the reference direction set_ha and set_va : see below

  • set_axis_direction : “left”, “right”, “bottom”, or “top”
  • set_rotation : angle with respect to the reference direction
  • set_ha and set_va : see below
  • AxisLabels’ methods (label)

set_axis_direction : “left”, “right”, “bottom”, or “top” set_rotation : angle with respect to the reference direction set_ha and set_va

  • set_axis_direction : “left”, “right”, “bottom”, or “top”
  • set_rotation : angle with respect to the reference direction
  • set_ha and set_va

Adjusting ticklabels alignment

Alignment of TickLabels are treated specially. See below

Demo Ticklabel Alignment

Adjusting pad

To change the pad between ticks and ticklabels

  1. ax.axis["left"].major_ticklabels.set_pad(10)

Or ticklabels and axis-label

  1. ax.axis["left"].label.set_pad(10)

Simple Axis Pad

GridHelper

To actually define a curvilinear coordinate, you have to use your own grid helper. A generalised version of grid helper class is supplied and this class should suffice in most of cases. A user may provide two functions which defines a transformation (and its inverse pair) from the curved coordinate to (rectilinear) image coordinate. Note that while ticks and grids are drawn for curved coordinate, the data transform of the axes itself (ax.transData) is still rectilinear (image) coordinate.

  1. from mpl_toolkits.axisartist.grid_helper_curvelinear \
  2.      import GridHelperCurveLinear
  3. from mpl_toolkits.axisartist import Subplot
  5. # from curved coordinate to rectlinear coordinate.
  6. def tr(x, y):
  7.     x, y = np.asarray(x), np.asarray(y)
  8.     return x, y-x
  10. # from rectlinear coordinate to curved coordinate.
  11. def inv_tr(x,y):
  12.     x, y = np.asarray(x), np.asarray(y)
  13.     return x, y+x
  15. grid_helper = GridHelperCurveLinear((tr, inv_tr))
  17. ax1 = Subplot(fig, 1, 1, 1, grid_helper=grid_helper)
  19. fig.add_subplot(ax1)

You may use matplotlib’s Transform instance instead (but a inverse transformation must be defined). Often, coordinate range in a curved coordinate system may have a limited range, or may have cycles. In those cases, a more customized version of grid helper is required.

  1. import mpl_toolkits.axisartist.angle_helper as angle_helper
  3. # PolarAxes.PolarTransform takes radian. However, we want our coordinate
  4. # system in degree
  5. tr = Affine2D().scale(np.pi/180., 1.) + PolarAxes.PolarTransform()
  7. # extreme finder :  find a range of coordinate.
  8. # 20, 20 : number of sampling points along x, y direction
  9. # The first coordinate (longitude, but theta in polar)
  10. #   has a cycle of 360 degree.
  11. # The second coordinate (latitude, but radius in polar)  has a minimum of 0
  12. extreme_finder = angle_helper.ExtremeFinderCycle(20, 20,
  13.                                                  lon_cycle = 360,
  14.                                                  lat_cycle = None,
  15.                                                  lon_minmax = None,
  16.                                                  lat_minmax = (0, np.inf),
  17.                                                  )
  19. # Find a grid values appropriate for the coordinate (degree,
  20. # minute, second). The argument is a approximate number of grids.
  21. grid_locator1 = angle_helper.LocatorDMS(12)
  23. # And also uses an appropriate formatter.  Note that,the
  24. # acceptable Locator and Formatter class is a bit different than
  25. # that of mpl's, and you cannot directly use mpl's Locator and
  26. # Formatter here (but may be possible in the future).
  27. tick_formatter1 = angle_helper.FormatterDMS()
  29. grid_helper = GridHelperCurveLinear(tr,
  30.                                     extreme_finder=extreme_finder,
  31.                                     grid_locator1=grid_locator1,
  32.                                     tick_formatter1=tick_formatter1
  33.                                     )

Again, the transData of the axes is still a rectilinear coordinate (image coordinate). You may manually do conversion between two coordinates, or you may use Parasite Axes for convenience.:

  1. ax1 = SubplotHost(fig, 1, 2, 2, grid_helper=grid_helper)
  3. # A parasite axes with given transform
  4. ax2 = ParasiteAxesAuxTrans(ax1, tr, "equal")
  5. # note that ax2.transData == tr + ax1.transData
  6. # Anything you draw in ax2 will match the ticks and grids of ax1.
  7. ax1.parasites.append(ax2)

Demo Curvelinear Grid

FloatingAxis

A floating axis is an axis one of whose data coordinate is fixed, i.e, its location is not fixed in Axes coordinate but changes as axes data limits changes. A floating axis can be created using new_floating_axis method. However, it is your responsibility that the resulting AxisArtist is properly added to the axes. A recommended way is to add it as an item of Axes’s axis attribute.:

  1. # floating axis whose first (index starts from 0) coordinate
  2. # (theta) is fixed at 60
  4. ax1.axis["lat"] = axis = ax1.new_floating_axis(0, 60)
  5. axis.label.set_text(r"$\theta = 60^{\circ}$")
  6. axis.label.set_visible(True)

See the first example of this page.

Current Limitations and TODO’s

The code need more refinement. Here is a incomplete list of issues and TODO’s

  • No easy way to support a user customized tick location (for curvilinear grid). A new Locator class needs to be created.
  • FloatingAxis may have coordinate limits, e.g., a floating axis of x = 0, but y only spans from 0 to 1.
  • The location of axislabel of FloatingAxis needs to be optionally given as a coordinate value. ex, a floating axis of x=0 with label at y=1

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