x轴上带有时间戳的 pandas 图-刻度单位? [英] pandas plot with timestamps on x axis - tick units?
问题描述
我有一个带有日期时间值的熊猫数据框:
I have a pandas data frame with datetime values:
Date/Time Event Value
0 2018-11-15 17:17:49 62
1 2018-11-15 17:27:50 63
2 2018-11-15 17:37:50 64
3 2018-11-15 17:42:49 65
4 2018-11-15 18:17:49 64
5 2018-11-15 19:27:48 65
6 2018-11-15 19:37:48 66
7 2018-11-15 19:47:49 67
8 2018-11-15 19:57:49 68
9 2018-11-15 20:12:49 69
print (tmp.dtypes)
Date/Time datetime64[ns]
Event Value int64
按如下所示进行绘制:
fig, ax1 = plt.subplots(1,1,figsize=(8,4))
tmp.plot(x='Date/Time',y='Event Value', legend=None, ax=ax1)
x轴已使用适当的日期和时间进行了格式化(并且可以使用set_major_formatter进行进一步格式化).
The x-axis has been formatted with appropriate dates and times (and can be further formatted using set_major_formatter).
现在，我想在图表上绘制一个矩形，以突出显示特定时间.为此，我需要知道x位置.
Now I want to draw a rectangle on the chart, to highlight a particular time. To do that I need to know the x-positions.
print([x for x in ax1.get_xticks()])
[737013.7291666666, 737013.75, 737013.7708333334, 737013.7916666666, 737013.8125, 737013.8333333334]
这些确实是x值，即，如果我将它们用作矩形的原点，则会按预期绘制.
These are indeed the x-values, i.e. if I use them as origins for the rectangle it plots as expected.
rect = patches.Rectangle((737013.75,55),.01,100)
ax1.add_patch(rect)
我不知道这些数字是从哪里来的.它们不是时代值，尽管看起来有点像它们("737013.75"对应于"1970-01-09 07:43:33"，对我而言没有任何意义).
I don't understand where these numbers come from. They're not epoch values, though they look a little bit like them ('737013.75' corresponds to '1970-01-09 07:43:33', which doesn't mean anything to me).
如何从数据框中的日期/时间值获取图表上的x位置?
How can I get the x-position on the chart from a Date/Time value in my dataframe?
推荐答案
如果使用x_compat=True选项绘制数据框，则可以确定单位为matplotlib.dates单位.否则，大熊猫可能会为您决定一些单位.
If you plot a dataframe with the x_compat=True option you can be certain that the units are matplotlib.dates units. Else, pandas may decide some units for you.
此处，熊猫使用matplotlib日期单位.它们被定义为:
Here, pandas uses matplotlib dates units. They are defined as:
Matplotlib使用浮点数表示日期，该浮点数指定自0001-01-01 UTC以来的天数加1.例如，0001-01-01、06:00为1.25，而不是0.25.值< 1，即不支持UTC 0001-01-01之前的日期.
Matplotlib represents dates using floating point numbers specifying the number of days since 0001-01-01 UTC, plus 1. For example, 0001-01-01, 06:00 is 1.25, not 0.25. Values < 1, i.e. dates before 0001-01-01 UTC are not supported.
但是，您不希望自己计算这些单位. Matplotlib提供辅助功能
You are however not expected to calculate those units yourself. Matplotlib provides helper functions
-
matplotlib.dates.date2num -
matplotlib.dates.num2date -
matplotlib.dates.datestr2num
matplotlib.dates.date2nummatplotlib.dates.num2datematplotlib.dates.datestr2num
，可用于在单位之间来回计算.
which you can use to calculate back and forth between the units.
例如，matplotlib.dates.datestr2num("2018-11-15 19:27:48")给您737013.8109722222.
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