# matplotlib.axes.Axes.fill_betweenx() in Python

Matplotlib is a library in Python and it is numerical – mathematical extension for NumPy library. The Axes Class contains most of the figure elements: Axis, Tick, Line2D, Text, Polygon, etc., and sets the coordinate system. And the instances of Axes supports callbacks through a callbacks attribute.

## matplotlib.axes.Axes.fill_betweenx() Function

The Axes.fill_betweenx() function in axes module of matplotlib library is used to fill the area between two vertical curves.
Syntax: Axes.fill_betweenx(self, y, x1, x2=0, where=None, step=None, interpolate=False, *, data=None, **kwargs) Parameters: This method accept the following parameters that are described below:
• y: This parameter contains the y coordinates of the data points which are used to define the curves.
• x1: This parameter contains the x coordinates of the data points which are used to define the first curves
• x2: This parameter contains the x coordinates of the data points which are used to define the second curves. It is an optional with default value of 0.
• where: This parameter is an optional parameter. And it is used to exclude some horizontal regions from being filled.
• interpolate: This parameter is also an optional parameter. And it is the linewidth of the errorbar lines with default value NONE.
• step: This parameter is also an optional parameter. And it is used to define if the filling should be a step function or not.
Returns: This returns a PolyCollection containing the plotted polygons.
Below examples illustrate the matplotlib.axes.Axes.fill_betweenx() function in matplotlib.axes: Example-1:
 # Implementation of matplotlib function        import numpy as np import matplotlib.pyplot as plt    y = np.arange(-5, 5, 0.01) x1 = -y * 2 + y + 10x2 = 2 * y + y    fig, ax = plt.subplots() ax.plot(y, x1, y, x2, color ='black') ax.fill_betweenx(y, x1, x2, where = x2 >x1,                   facecolor ='green', alpha = 0.8)   ax.fill_betweenx(y, x1, x2, where = x2 <= x1,                  facecolor ='black', alpha = 0.8)    ax.set_title('matplotlib.axes.Axes.fill_betweenx Example1') plt.show()

Output: Example-2:
 # Implementation of matplotlib function       import numpy as np import matplotlib.pyplot as plt   y = np.arange(0.0, 2, 0.01) x1 = np.sin(2 * np.pi * y) x2 = 0.8 * np.sin(4 * np.pi * y)   fig, [ax1, ax2, ax3, ax4] = plt.subplots(1, 4,                                          sharey = True,                                          figsize =(6, 6))   ax1.fill_betweenx(y, 0, x1, facecolor ='green') ax1.set_title('Fill_Betweenx x1 and 0')   ax2.fill_betweenx(y, x1, 1, facecolor ='green') ax2.set_title('Fill_Betweenx x1 and 1')   ax3.fill_betweenx(y, x1, x2, facecolor ='green') ax3.set_title('Fill_Betweenx x1 and y2')   ax4.fill_betweenx(y, x1, x2, where = x2 <= x1,                    facecolor ='green')   ax4.set_title('Fill_Between x1 and x2 with x2<= x1 ' ) plt.show()

Output:

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