python – 是否有可能在matplotlib中获得曲线下的颜色渐变？

大多数先前的答案建议在pcolormesh填充上绘制白色多边形.由于两个原因,这不太理想：

>轴的背景不能是透明的,因为它上面有一个填充的多边形
> pcolormesh绘制速度相当慢,并且不能平滑插值.

这是一个更多的工作,但有一种方法可以更快地绘制并提供更好的视觉效果：设置用imshow绘制的图像的剪辑路径.

举个例子：

import numpy as np
import matplotlib.pyplot as plt
import matplotlib.colors as mcolors
from matplotlib.patches import Polygon
np.random.seed(1977)

def main():
    for _ in range(5):
        gradient_fill(*generate_data(100))
    plt.show()

def generate_data(num):
    x = np.linspace(0,100,num)
    y = np.random.normal(0,1,num).cumsum()
    return x,y

def gradient_fill(x,y,fill_color=None,ax=None,**kwargs):
    """
    Plot a line with a linear alpha gradient filled beneath it.

    Parameters
    ----------
    x,y : array-like
        The data values of the line.
    fill_color : a matplotlib color specifier (string,tuple) or None
        The color for the fill. If None,the color of the line will be used.
    ax : a matplotlib Axes instance
        The axes to plot on. If None,the current pyplot axes will be used.
    Additional arguments are passed on to matplotlib's ``plot`` function.

    Returns
    -------
    line : a Line2D instance
        The line plotted.
    im : an AxesImage instance
        The transparent gradient clipped to just the area beneath the curve.
    """
    if ax is None:
        ax = plt.gca()

    line,= ax.plot(x,**kwargs)
    if fill_color is None:
        fill_color = line.get_color()

    zorder = line.get_zorder()
    alpha = line.get_alpha()
    alpha = 1.0 if alpha is None else alpha

    z = np.empty((100,4),dtype=float)
    rgb = mcolors.colorConverter.to_rgb(fill_color)
    z[:,:,:3] = rgb
    z[:,-1] = np.linspace(0,alpha,100)[:,None]

    xmin,xmax,ymin,ymax = x.min(),x.max(),y.min(),y.max()
    im = ax.imshow(z,aspect='auto',extent=[xmin,ymax],origin='lower',zorder=zorder)

    xy = np.column_stack([x,y])
    xy = np.vstack([[xmin,ymin],xy,[xmax,[xmin,ymin]])
    clip_path = Polygon(xy,facecolor='none',edgecolor='none',closed=True)
    ax.add_patch(clip_path)
    im.set_clip_path(clip_path)

    ax.autoscale(True)
    return line,im

main()