How to set 'y > 0' formula in set_xlim of matplotlib?

Question

I want to set x range according to y value in plotting graph such as y > 0 but I'm not sure how to set this one. Could you let me know how to set it?

df = pd.read_csv(file.csv)
x = np.array(df1['A'])
y = np.array(df1['B'])
z = np.array(df1['C'])

x_for_ax1 = np.ma.masked_where((y < 0) | (y > 100), x)

fig, (ax2, ax1) = plt.subplots(ncols=1, nrows=2)

# range of ax1.set_xlim and ax1.set_xlim is same. 
ax1.set_ylim([-10, 40])
ax2.set_ylim([-5, 5])
ax1.set_xlim([x_for_ax1.min(), x_for_ax1.max()])
ax2.set_xlim([x_for_ax1.min(), x_for_ax1.max()])

Answer 1

If you want to set the x-limits to the range of the y-axis, you can use a masked array and get its minimum and maximum.

In the example below, at the left both subplots get the x-limits where either y or z are in range. At the right, each subplot only gets the x-range where its corresponding y is in range.

For demonstration purposes, the example creates a data frame from some dummy data.

import matplotlib.pyplot as plt
import numpy as np
import pandas as pd

a = np.linspace(-1, 4, 500)
b = np.sin(a) * 100
c = np.cos(a) * 150
df = pd.DataFrame({'A': a, 'B': b, 'C': c})
x = np.array(df['A'])
y = np.array(df['B'])
z = np.array(df['C'])

fig, ((ax1, ax3),(ax2, ax4)) = plt.subplots(ncols=2, nrows=2)

ax1.set_xlabel('x')
ax2.set_xlabel('x')
ax3.set_xlabel('x')
ax4.set_xlabel('x')
ax1.set_ylabel('y')
ax3.set_ylabel('y')
ax2.set_ylabel('z')
ax4.set_ylabel('z')

ymin = 1
ymax = 100
zmin = 1
zmax = 150
x_for_ax1 = np.ma.masked_where(((y < ymin) | (y > ymax)) & ((z < zmin) | (z > zmax)), x)
x_for_ax3 = np.ma.masked_where((y < ymin) | (y > ymax), x)
x_for_ax4 = np.ma.masked_where((z < zmin) | (z > zmax), x)

ax1.plot(x, y)
ax3.plot(x, y)
ax1.set_ylim([ymin, ymax])
ax3.set_ylim([ymin, ymax])

ax2.plot(x, z)
ax4.plot(x, z)
ax2.set_ylim([zmin, zmax])
ax4.set_ylim([zmin, zmax])

ax1.set_xlim([x_for_ax1.min(), x_for_ax1.max()])
ax2.set_xlim([x_for_ax1.min(), x_for_ax1.max()])
ax1.set_title('x limited to y and z range')
ax2.set_title('x limited to y and z range')

ax3.set_xlim([x_for_ax3.min(), x_for_ax3.max()])
ax3.set_title('x limited to y range')

ax4.set_xlim([x_for_ax4.min(), x_for_ax4.max()])
ax4.set_title('x limited to z range')

plt.tight_layout(w_pad=1)
plt.show()