python - How to extract points from a graph?

Question

I have a question.

I have plotted a graph using Matplotlib like this:

from matplotlib import pyplot
import numpy
from scipy.interpolate import spline

widths = numpy.array([0, 30, 60, 90, 120, 150, 180])
heights = numpy.array([26, 38.5, 59.5, 82.5, 120.5, 182.5, 319.5])

xnew = numpy.linspace(widths.min(),widths.max(),300)
heights_smooth = spline(widths,heights,xnew)

pyplot.plot(xnew,heights_smooth)
pyplot.show()

Now I want to query a height value using width value as an argument. I cannot seem to find how to do that. Please help! Thanks in advance!

Answer 1

plot() returns a useful object: [<matplotlib.lines.Line2D object at 0x38c9910>]
From that we can get x- and y-axis values:

import matplotlib.pyplot as plt, numpy as np
...
line2d = plt.plot(xnew,heights_smooth)
xvalues = line2d[0].get_xdata()
yvalues = line2d[0].get_ydata()

Then we can get the index of one of the width values:

idx = np.where(xvalues==xvalues[-2]) # this is 179.3979933110368
# idx is a tuple of array(s) containing index where value was found
# in this case -> (array([298]),)

And the corresponding height:

yvalues[idx]
# -> array([ 315.53469])

To check we can use get_xydata():

>>> xy = line2d[0].get_xydata()
>>> xy[-2]
array([ 179.39799331,  315.53469   ])