matplotlib: add scale bars to axes

scalebars.py

# -*- coding: utf-8 -*-
# -*- mode: python -*-
# Adapted from mpl_toolkits.axes_grid2
# LICENSE: Python Software Foundation (http://docs.python.org/license.html)

from matplotlib.offsetbox import AnchoredOffsetbox
class AnchoredScaleBar(AnchoredOffsetbox):
    def __init__(self, transform, sizex=0, sizey=0, labelx=None, labely=None, loc=4,
                 pad=0.1, borderpad=0.1, sep=2, prop=None, **kwargs):
        """
        Draw a horizontal and/or vertical  bar with the size in data coordinate
        of the give axes. A label will be drawn underneath (center-aligned).

        - transform : the coordinate frame (typically axes.transData)
        - sizex,sizey : width of x,y bar, in data units. 0 to omit
        - labelx,labely : labels for x,y bars; None to omit
        - loc : position in containing axes
        - pad, borderpad : padding, in fraction of the legend font size (or prop)
        - sep : separation between labels and bars in points.
        - **kwargs : additional arguments passed to base class constructor
        """
        from matplotlib.patches import Rectangle
        from matplotlib.offsetbox import AuxTransformBox, VPacker, HPacker, TextArea, DrawingArea
        bars = AuxTransformBox(transform)
        if sizex:
            bars.add_artist(Rectangle((0,0), sizex, 0, fc="none"))
        if sizey:
            bars.add_artist(Rectangle((0,0), 0, sizey, fc="none"))

        if sizex and labelx:
            bars = VPacker(children=[bars, TextArea(labelx, minimumdescent=False)],
                           align="center", pad=0, sep=sep)
        if sizey and labely:
            bars = HPacker(children=[TextArea(labely), bars],
                            align="center", pad=0, sep=sep)

        AnchoredOffsetbox.__init__(self, loc, pad=pad, borderpad=borderpad,
                                   child=bars, prop=prop, frameon=False, **kwargs)

def add_scalebar(ax, matchx=True, matchy=True, hidex=True, hidey=True, **kwargs):
    """ Add scalebars to axes

    Adds a set of scale bars to *ax*, matching the size to the ticks of the plot
    and optionally hiding the x and y axes

    - ax : the axis to attach ticks to
    - matchx,matchy : if True, set size of scale bars to spacing between ticks
                    if False, size should be set using sizex and sizey params
    - hidex,hidey : if True, hide x-axis and y-axis of parent
    - **kwargs : additional arguments passed to AnchoredScaleBars

    Returns created scalebar object
    """
    def f(axis):
        l = axis.get_majorticklocs()
        return len(l)>1 and (l[1] - l[0])
    
    if matchx:
        kwargs['sizex'] = f(ax.xaxis)
        kwargs['labelx'] = str(kwargs['sizex'])
    if matchy:
        kwargs['sizey'] = f(ax.yaxis)
        kwargs['labely'] = str(kwargs['sizey'])
        
    sb = AnchoredScaleBar(ax.transData, **kwargs)
    ax.add_artist(sb)

    if hidex : ax.xaxis.set_visible(False)
    if hidey : ax.yaxis.set_visible(False)

    return sb