You are reading an old version of the documentation (v2.1.2). For the latest version see https://matplotlib.org/stable/gallery/lines_bars_and_markers/fill_between_demo.html

This example shows how to use `fill_between`

to color between lines based on
user-defined logic.

```
fig, (ax1, ax2, ax3) = plt.subplots(3, 1, sharex=True)
ax1.fill_between(x, 0, y1)
ax1.set_ylabel('between y1 and 0')
ax2.fill_between(x, y1, 1)
ax2.set_ylabel('between y1 and 1')
ax3.fill_between(x, y1, y2)
ax3.set_ylabel('between y1 and y2')
ax3.set_xlabel('x')
```

Now fill between y1 and y2 where a logical condition is met. Note
this is different than calling
`fill_between(x[where], y1[where], y2[where] ...)`

because of edge effects over multiple contiguous regions.

```
fig, (ax, ax1) = plt.subplots(2, 1, sharex=True)
ax.plot(x, y1, x, y2, color='black')
ax.fill_between(x, y1, y2, where=y2 >= y1, facecolor='green', interpolate=True)
ax.fill_between(x, y1, y2, where=y2 <= y1, facecolor='red', interpolate=True)
ax.set_title('fill between where')
# Test support for masked arrays.
y2 = np.ma.masked_greater(y2, 1.0)
ax1.plot(x, y1, x, y2, color='black')
ax1.fill_between(x, y1, y2, where=y2 >= y1,
facecolor='green', interpolate=True)
ax1.fill_between(x, y1, y2, where=y2 <= y1,
facecolor='red', interpolate=True)
ax1.set_title('Now regions with y2>1 are masked')
```

This example illustrates a problem; because of the data gridding, there are undesired unfilled triangles at the crossover points. A brute-force solution would be to interpolate all arrays to a very fine grid before plotting.

Use transforms to create axes spans where a certain condition is satisfied:

```
fig, ax = plt.subplots()
y = np.sin(4 * np.pi * x)
ax.plot(x, y, color='black')
# use data coordinates for the x-axis and the axes coordinates for the y-axis
import matplotlib.transforms as mtransforms
trans = mtransforms.blended_transform_factory(ax.transData, ax.transAxes)
theta = 0.9
ax.axhline(theta, color='green', lw=2, alpha=0.5)
ax.axhline(-theta, color='red', lw=2, alpha=0.5)
ax.fill_between(x, 0, 1, where=y > theta,
facecolor='green', alpha=0.5, transform=trans)
ax.fill_between(x, 0, 1, where=y < -theta,
facecolor='red', alpha=0.5, transform=trans)
plt.show()
```

**Total running time of the script:** ( 0 minutes 0.115 seconds)