Matplotlib Ref Fill_Between

Title: Matplotlib fill_between() / fill_betweenx() Function

URL Source: https://example.com/matplotlib/matplotlib-ref-fill_between.html

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fill_between() is used to fill the area between two horizontal curves, and fill_betweenx() fills the area between two vertical curves.

Commonly used for visualizing confidence intervals, uncertainty ranges, difference areas, etc.

Function Definition

matplotlib.pyplot.fill_between(x, y1, y2=0, where=None, interpolate=False, step=None, **kwargs)matplotlib.pyplot.fill_betweenx(y, x1, x2=0, where=None, interpolate=False, step=None, **kwargs)

Parameter Description

\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n

Parameter	Type	Description
x / y	array	Data points for the first coordinate axis
y1, y2 / x1, x2	array or scalar	Two curves. Scalar represents a constant line. y2 defaults to 0, filling to the x-axis
where	array of bool	Specifies which intervals need to be filled. True fills, False does not fill
interpolate	bool	If True, only interpolates exact boundaries at curve intersection points
step	str	Step filling: 'pre', 'post', 'mid'
color / facecolor	color	Fill color
alpha	float	Transparency
label	str	Legend label
hatch	str	Fill pattern: '/', '\\', '\|', '-', '+', 'x', '*'

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fill_between and fill_betweenx return a PolyCollection object, which can be used for further modifications or passed to colorbar.
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Usage Examples

Example 1: Fill to x-axis (default y2=0)

Example

import matplotlib.pyplot as plt\n\nimport numpy as np\n\nx = np.linspace(0,10,100)\n\n y = np.sin(x)\n\nfig, ax = plt.subplots(layout='constrained')\n\n# Plot the main curve\n\n ax.plot(x, y,'b-', linewidth=2, label='sin(x)')\n\n# Fill the area between the curve and the x-axis\n\n ax.fill_between(x, y, alpha=0.3, color='blue',\n\n label='Area under curve')\n\nax.set_title('fill_between: Area Between Curve and X-axis')\n\n ax.set_xlabel('x')\n\n ax.set_ylabel('sin(x)')\n\n ax.legend()\n\n ax.grid(True, alpha=0.3)\n\n plt.show()

Example 2: Confidence Interval (area between two curves)

Example

import matplotlib.pyplot as plt\n\nimport numpy as np\n\nx = np.linspace(0,10,100)\n\n y_mean = np.sin(x) + 5# Mean line\n\n y_std =0.3 + 0.1 * x # Standard deviation increases with x\n\n# Upper and lower bounds\n\n y_upper = y_mean + y_std\n\n y_lower = y_mean - y_std\n\nfig, ax = plt.subplots(figsize=(8,5), layout='constrained')\n\n# Fill confidence intervals on both sides of the mean\n\n ax.fill_between(x, y_lower, y_upper,\n\n alpha=0.3, color='steelblue',\n\n label='Confidence Interval')\n\n# Plot Mean line\n\n ax.plot(x, y_mean,'steelblue', linewidth=2, label='Mean')\n\n# Plot upper and lower bounds\n\n ax.plot(x, y_upper,'steelblue', linewidth=0.5, alpha=0.5)\n\n ax.plot(x, y_lower,'steelblue', linewidth=0.5, alpha=0.5)\n\nax.set_title('Confidence Interval (fill_between)')\n\n ax.set_xlabel('x')\n\n ax.set_ylabel('y')\n\n ax.legend()\n\n ax.grid(True, alpha=0.3)\n\n plt.show()

Example 3: where Parameter Conditional Fill

Example

import matplotlib.pyplot as plt\n\nimport numpy as np\n\nx = np.linspace(0,10,200)\n\n y1 = np.sin(x)\n\n y2 = np.cos(x)\n\nfig, ax = plt.subplots(figsize=(8,5), layout='constrained')\n\nax.plot(x, y1,'blue', label='sin(x)')\n\n ax.plot(x, y2,'red', label='cos(x)')\n\n# only at sin(x) >= cos(x) interval fill for\n\n ax.fill_between(x, y1, y2,\n\n where=(y1 >= y2),# Condition: fill when y1 is above y2\n\n color='blue', alpha=0.3,\n\n label='sin ≥ cos')\n\n# only at sin(x) < cos(x) interval fill for（different colors)\n\n ax.fill_between(x, y1, y2,\n\n where=(y1 < y2),\n\n color='red', alpha=0.3,\n\n label='sin < cos')\n\nax.set_title('Conditional Fill with where Parameter')\n\n ax.set_xlabel('x')\n\n ax.legend()\n\n ax.grid(True, alpha=0.3)\n\n plt.show()

Example 4: fill_betweenx Vertical Fill

Example

import matplotlib.pyplot as plt\n\nimport numpy as np\n\n# fill_betweenx The parameter order is (y, x1, x2)\n\n y = np.linspace(0,10,100)\n\n x1 =2 +

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