注意
前往末尾以下載完整範例程式碼。
帶註解的熱圖#
通常需要將依賴兩個獨立變數的資料顯示為顏色編碼的影像圖。這通常稱為熱圖。如果資料是分類的，則稱為分類熱圖。
Matplotlib 的 imshow 函數使得產生此類圖表特別容易。
以下範例展示如何建立帶有註解的熱圖。我們將從一個簡單的範例開始，並將其擴展為可用的通用函數。
一個簡單的分類熱圖#
我們可以先定義一些資料。我們需要的是一個 2D 列表或陣列，用於定義要進行顏色編碼的資料。然後我們還需要兩個類別的列表或陣列；當然，這些列表中的元素數需要與沿各自軸的資料相符。熱圖本身是一個 imshow 圖，其標籤設定為我們擁有的類別。請注意，設定刻度位置 (set_xticks) 和刻度標籤 (set_xticklabels) 非常重要，否則它們會不同步。位置只是遞增的整數，而刻度標籤是要顯示的標籤。最後，我們可以藉由建立 Text，在每個單元格中顯示該單元格的值，來標記資料本身。
import matplotlib.pyplot as plt
import numpy as np
import matplotlib
import matplotlib as mpl
vegetables = ["cucumber", "tomato", "lettuce", "asparagus",
"potato", "wheat", "barley"]
farmers = ["Farmer Joe", "Upland Bros.", "Smith Gardening",
"Agrifun", "Organiculture", "BioGoods Ltd.", "Cornylee Corp."]
harvest = np.array([[0.8, 2.4, 2.5, 3.9, 0.0, 4.0, 0.0],
[2.4, 0.0, 4.0, 1.0, 2.7, 0.0, 0.0],
[1.1, 2.4, 0.8, 4.3, 1.9, 4.4, 0.0],
[0.6, 0.0, 0.3, 0.0, 3.1, 0.0, 0.0],
[0.7, 1.7, 0.6, 2.6, 2.2, 6.2, 0.0],
[1.3, 1.2, 0.0, 0.0, 0.0, 3.2, 5.1],
[0.1, 2.0, 0.0, 1.4, 0.0, 1.9, 6.3]])
fig, ax = plt.subplots()
im = ax.imshow(harvest)
# Show all ticks and label them with the respective list entries
ax.set_xticks(range(len(farmers)), labels=farmers,
rotation=45, ha="right", rotation_mode="anchor")
ax.set_yticks(range(len(vegetables)), labels=vegetables)
# Loop over data dimensions and create text annotations.
for i in range(len(vegetables)):
for j in range(len(farmers)):
text = ax.text(j, i, harvest[i, j],
ha="center", va="center", color="w")
ax.set_title("Harvest of local farmers (in tons/year)")
fig.tight_layout()
plt.show()
使用輔助函數程式碼樣式#
如在程式碼樣式 中討論的，人們可能希望重複使用此類程式碼，以針對不同輸入資料和/或在不同軸上建立某種熱圖。我們建立一個函數，該函數將資料以及行和列標籤作為輸入，並允許用於自訂圖形的引數
在這裡，除了上述之外，我們還希望建立色彩條，並將標籤放置在熱圖上方而不是下方。註解應根據閾值取得不同的顏色，以獲得更好的對比度，與像素顏色形成對比。最後，我們關閉周圍軸的脊柱，並建立白色線條的格線以分隔單元格。
def heatmap(data, row_labels, col_labels, ax=None,
cbar_kw=None, cbarlabel="", **kwargs):
"""
Create a heatmap from a numpy array and two lists of labels.
Parameters
----------
data
A 2D numpy array of shape (M, N).
row_labels
A list or array of length M with the labels for the rows.
col_labels
A list or array of length N with the labels for the columns.
ax
A `matplotlib.axes.Axes` instance to which the heatmap is plotted. If
not provided, use current Axes or create a new one. Optional.
cbar_kw
A dictionary with arguments to `matplotlib.Figure.colorbar`. Optional.
cbarlabel
The label for the colorbar. Optional.
**kwargs
All other arguments are forwarded to `imshow`.
"""
if ax is None:
ax = plt.gca()
if cbar_kw is None:
cbar_kw = {}
# Plot the heatmap
im = ax.imshow(data, **kwargs)
# Create colorbar
cbar = ax.figure.colorbar(im, ax=ax, **cbar_kw)
cbar.ax.set_ylabel(cbarlabel, rotation=-90, va="bottom")
# Show all ticks and label them with the respective list entries.
ax.set_xticks(range(data.shape[1]), labels=col_labels,
rotation=-30, ha="right", rotation_mode="anchor")
ax.set_yticks(range(data.shape[0]), labels=row_labels)
# Let the horizontal axes labeling appear on top.
ax.tick_params(top=True, bottom=False,
labeltop=True, labelbottom=False)
# Turn spines off and create white grid.
ax.spines[:].set_visible(False)
ax.set_xticks(np.arange(data.shape[1]+1)-.5, minor=True)
ax.set_yticks(np.arange(data.shape[0]+1)-.5, minor=True)
ax.grid(which="minor", color="w", linestyle='-', linewidth=3)
ax.tick_params(which="minor", bottom=False, left=False)
return im, cbar
def annotate_heatmap(im, data=None, valfmt="{x:.2f}",
textcolors=("black", "white"),
threshold=None, **textkw):
"""
A function to annotate a heatmap.
Parameters
----------
im
The AxesImage to be labeled.
data
Data used to annotate. If None, the image's data is used. Optional.
valfmt
The format of the annotations inside the heatmap. This should either
use the string format method, e.g. "$ {x:.2f}", or be a
`matplotlib.ticker.Formatter`. Optional.
textcolors
A pair of colors. The first is used for values below a threshold,
the second for those above. Optional.
threshold
Value in data units according to which the colors from textcolors are
applied. If None (the default) uses the middle of the colormap as
separation. Optional.
**kwargs
All other arguments are forwarded to each call to `text` used to create
the text labels.
"""
if not isinstance(data, (list, np.ndarray)):
data = im.get_array()
# Normalize the threshold to the images color range.
if threshold is not None:
threshold = im.norm(threshold)
else:
threshold = im.norm(data.max())/2.
# Set default alignment to center, but allow it to be
# overwritten by textkw.
kw = dict(horizontalalignment="center",
verticalalignment="center")
kw.update(textkw)
# Get the formatter in case a string is supplied
if isinstance(valfmt, str):
valfmt = matplotlib.ticker.StrMethodFormatter(valfmt)
# Loop over the data and create a `Text` for each "pixel".
# Change the text's color depending on the data.
texts = []
for i in range(data.shape[0]):
for j in range(data.shape[1]):
kw.update(color=textcolors[int(im.norm(data[i, j]) > threshold)])
text = im.axes.text(j, i, valfmt(data[i, j], None), **kw)
texts.append(text)
return texts
現在，以上內容使我們可以讓實際的圖形建立保持相當緊湊。
fig, ax = plt.subplots()
im, cbar = heatmap(harvest, vegetables, farmers, ax=ax,
cmap="YlGn", cbarlabel="harvest [t/year]")
texts = annotate_heatmap(im, valfmt="{x:.1f} t")
fig.tight_layout()
plt.show()
一些更複雜的熱圖範例#
在以下內容中，我們透過在不同情況下套用先前建立的函數，並使用不同的引數來展示其多功能性。
np.random.seed(19680801)
fig, ((ax, ax2), (ax3, ax4)) = plt.subplots(2, 2, figsize=(8, 6))
# Replicate the above example with a different font size and colormap.
im, _ = heatmap(harvest, vegetables, farmers, ax=ax,
cmap="Wistia", cbarlabel="harvest [t/year]")
annotate_heatmap(im, valfmt="{x:.1f}", size=7)
# Create some new data, give further arguments to imshow (vmin),
# use an integer format on the annotations and provide some colors.
data = np.random.randint(2, 100, size=(7, 7))
y = [f"Book {i}" for i in range(1, 8)]
x = [f"Store {i}" for i in list("ABCDEFG")]
im, _ = heatmap(data, y, x, ax=ax2, vmin=0,
cmap="magma_r", cbarlabel="weekly sold copies")
annotate_heatmap(im, valfmt="{x:d}", size=7, threshold=20,
textcolors=("red", "white"))
# Sometimes even the data itself is categorical. Here we use a
# `matplotlib.colors.BoundaryNorm` to get the data into classes
# and use this to colorize the plot, but also to obtain the class
# labels from an array of classes.
data = np.random.randn(6, 6)
y = [f"Prod. {i}" for i in range(10, 70, 10)]
x = [f"Cycle {i}" for i in range(1, 7)]
qrates = list("ABCDEFG")
norm = matplotlib.colors.BoundaryNorm(np.linspace(-3.5, 3.5, 8), 7)
fmt = matplotlib.ticker.FuncFormatter(lambda x, pos: qrates[::-1][norm(x)])
im, _ = heatmap(data, y, x, ax=ax3,
cmap=mpl.colormaps["PiYG"].resampled(7), norm=norm,
cbar_kw=dict(ticks=np.arange(-3, 4), format=fmt),
cbarlabel="Quality Rating")
annotate_heatmap(im, valfmt=fmt, size=9, fontweight="bold", threshold=-1,
textcolors=("red", "black"))
# We can nicely plot a correlation matrix. Since this is bound by -1 and 1,
# we use those as vmin and vmax. We may also remove leading zeros and hide
# the diagonal elements (which are all 1) by using a
# `matplotlib.ticker.FuncFormatter`.
corr_matrix = np.corrcoef(harvest)
im, _ = heatmap(corr_matrix, vegetables, vegetables, ax=ax4,
cmap="PuOr", vmin=-1, vmax=1,
cbarlabel="correlation coeff.")
def func(x, pos):
return f"{x:.2f}".replace("0.", ".").replace("1.00", "")
annotate_heatmap(im, valfmt=matplotlib.ticker.FuncFormatter(func), size=7)
plt.tight_layout()
plt.show()
參考
本範例中顯示了以下函數、方法、類別和模組的使用
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