How to plot discrete point with matplotlib?

[YanXingYu-star]

I am doing some calculation with pint and then I need to plot them with matplotlib.

For a array of data (likes [1, 1, 1] ureg.liter), this is fine.

But when I link some independent points (likes [1 ureg.liter, 1ureg.liter]), it will return DimensionalityError: Cannot convert from 'liter' to 'dimensionless'

there is my code

import matplotlib.pyplot as plt
import numpy as np
import pint
ureg = UnitRegistry()
ureg.setup_matplotlib()

#some calculation......

va = 0.5 * ureg.L
vc = 0.05 *ureg.L
pa = 0.1013 * ureg.MPa
pb = 2*ureg.MPa
pc = 7*ureg.MPa

a_ = np.arange(180,361,10)
v =np.cos(np.radians(a_))
p = pa*(va)**1.34/(v**1.34)

fig, ax = plt.subplots()
ax.xaxis.set_units(ureg.L)
ax.yaxis.set_units(ureg.MPa)
ax.plot(v, p)  # draw the line -> right

ax.plot([vc,vc],[pb,pc])  #connect two point -> Wrong

Did I do some thing wrong? How can I connect the two points?

Thank you!

[dopplershift]

When you do ax.plot([vc,vc],[pb,pc]), Matplotlib is going to try to convert [ 0.05 * ureg.L, 0.05 * ureg.L] and [2 * ureg.MPa, 7 * ureg.MPa] to arrays, which is not going to do what you want (at best it will result in an array of objects). You want to create a single array to pass to plot, as you did on the line above. One way to do that would be:

ax.plot([vc.m, vc.m] * vc.units, [pb.m, pc.m_as(pb.units)] * pb.units)

[keewis]

another way would be to use ureg.force_ndarray_like = True (not sure if that's actually necessary) and call np.stack or np.concatenate on the input data:

ax.plot(np.stack([vc, vc]), np.stack([pb, pc]))

As far as I remember, that should automatically convert to consistent units.

[YanXingYu-star]

Thank you! these are all very helpful.