Adjust exponent text after setting scientific limits on matplotlib axis

Question

Building on @edsmith’s answer one possible work around which does what I’d like is to get the offset text, convert it to a latex string, turn off the offset and add in that string at the top of the axis.

def format_exponent(ax, axis="y"):

    # Change the ticklabel format to scientific format
    ax.ticklabel_format(axis=axis, style="sci", scilimits=(-2, 2))

    # Get the appropriate axis
    if axis == 'y':
        ax_axis = ax.yaxis
        x_pos = 0.0
        y_pos = 1.0
        horizontalalignment="left"
        verticalalignment="bottom"
    else:
        ax_axis = ax.xaxis
        x_pos = 1.0
        y_pos = -0.05
        horizontalalignment="right"
        verticalalignment="top"

    # Run plt.tight_layout() because otherwise the offset text doesn't update
    plt.tight_layout()
    ##### THIS IS A BUG 
    ##### Well, at least it's sub-optimal because you might not
    ##### want to use tight_layout(). If anyone has a better way of 
    ##### ensuring the offset text is updated appropriately
    ##### please comment!

    # Get the offset value
    offset = ax_axis.get_offset_text().get_text()

    if len(offset) > 0:
        # Get that exponent value and change it into latex format
        minus_sign = u'\u2212'
        expo = np.float(offset.replace(minus_sign, '-').split('e')[-1])
        offset_text = r'x$\mathregular{10^{%d}}$' %expo

        # Turn off the offset text that's calculated automatically
        ax_axis.offsetText.set_visible(False)

        # Add in a text box at the top of the y axis
        ax.text(x_pos, y_pos, offset_text, transform=ax.transAxes,
               horizontalalignment=horizontalalignment,
               verticalalignment=verticalalignment)
    return ax

Note that you should be able to use the position of the offset text by calling pos = ax_axis.get_offset_text().get_position() but these values are not in axis units (they’re likely pixel units – thanks @EdSmith – and thus not very helpful). Therefore I’ve just set the x_pos and y_pos values according to whichever axis we’re looking at.

I also wrote a little function to automatically detect appropriate x and y limits (even though I know that matplotlib has lots of fancy ways of doing this).

def get_min_max(x, pad=0.05):
    '''
    Find min and max values such that
    all the data lies within 90% of
    of the axis range
    '''
    r = np.max(x) - np.min(x)
    x_min = np.min(x) - pad * r
    x_max = np.max(x) + pad * r
    return x_min, x_max

So, to update my example from the question (with a slight change to make both axes need the exponent):

import matplotlib.pylab as plt
import numpy as np

# Create a figure and axis
fig, ax = plt.subplots()

# Plot 100 random points that are very small
x = np.random.rand(100)/100000.0
y = np.random.rand(100)/100000.0
ax.scatter(x, y)

# Set the x and y limits
x_min, x_max = get_min_max(x)
ax.set_xlim(x_min, x_max)
y_min, y_max = get_min_max(y)    
ax.set_ylim(y_min, y_max)

# Format the exponents nicely
ax = format_exponent(ax, axis="x")
ax = format_exponent(ax, axis="y")

# And show the figure
plt.show()

enter image description here

A gist with an ipython notebook showing the output of the code is available here.

I hope that helps!

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