Sets or returns the time interval at which data points will be displayed.
The value of this property determines how the chart's data will be displayed. For example, if the value is set to Month, then all of the data for a given month will be displayed in a single point on the chart. By contrast, if you set the value to Day, then data points that occur within the same month will be separated horizontally. This makes the display more fine-grained, whereas larger time units may indicate longer-term trends.
This corresponds to the "Base unit" property when configuring the category axis in Excel.
The default value for charts created in ExcelWriter is Days. If you wish to have a different BaseUnit for a newly created chart, you will need to set this property.