CODEWITHSUNDEEP
pythonmatplotlib
sircolinton
sircolinton

Reputation: 6846

How do I offset lines in matplotlib by X points

I'm using matplotlib to plot some data that I wish to annotate with arrows (distance markers). These arrows should be offset by several points so as not to overlap with the plotted data:

import matplotlib.pyplot as plt
import matplotlib.transforms as transforms

fig, ax = plt.subplots()

x = [0, 1]
y = [0, 0]

# Plot horizontal line
ax.plot(x, y)

dy = 5/72

offset = transforms.ScaledTranslation(0, dy, ax.get_figure().dpi_scale_trans)
verttrans = ax.transData+offset

# Plot horizontal line 5 points above (works!)
ax.plot(x, y, transform = verttrans)

# Draw arrow 5 points above line (doesn't work--not vertically translated)
ax.annotate("", (0,0), (1,0),
            size = 10,
            transform=verttrans,
            arrowprops = dict(arrowstyle = '<|-|>'))

plt.show()

Is there any way to make lines drawn by ax.annotate() be offset by X points? I wish to use absolute coordinates (e.g., points or inches) instead of data coordinates because the axis limits are prone to changing.

Thanks!

Upvotes: 4

Views: 4251

Answers (2)

sircolinton
sircolinton

Reputation: 6846

The following code does what I desired. It uses ax.transData and figure.get_dpi():

import matplotlib.pyplot as plt
import matplotlib.transforms as transforms

fig, ax = plt.subplots()


x = [0, 1]
y = [0, 0]


ax.plot(x, y)


dy = 5/72

i = 1  # 0 for dx

tmp = ax.transData.transform([(0,0), (1,1)])
tmp = tmp[1,i] - tmp[0,i]  # 1 unit in display coords
tmp = 1/tmp  # 1 pixel in display coords
tmp = tmp*dy*ax.get_figure().get_dpi()  # shift pixels in display coords

ax.plot(x, y)

ax.annotate("", [0,tmp], [1,tmp],
            size = 10,
            arrowprops = dict(arrowstyle = '<|-|>'))

plt.show()

Upvotes: 3

Alex Z
Alex Z

Reputation: 1539

What's your expected output? If you're just looking to move the arrow you're drawing vertically, the API for annotate is

annotate(s, xy, xytext=None, ...)

so you can draw something like

ax.annotate("", (0,0.01), (1,0.01),
    size = 10,
    arrowprops = dict(arrowstyle = '<|-|>'))

which is moved up by 0.01 in data coordinates in the y direction. You can also specify coordinates as a fraction of the total figure size in annotate (see doc). Is that what you wanted?

Upvotes: 2

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