matplotlib.axes.Axes.format_xdata
- 
Axes.format_xdata(self, x) - 
Return x formatted as an x-value.
This function will use the
fmt_xdataattribute if it is not None, else will fall back on the xaxis major formatter. 
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    https://matplotlib.org/3.2.2/api/_as_gen/matplotlib.axes.Axes.format_xdata.html