Any charts (except of course XY diagrams) use - in the normal case - a row or
column to derive their x axis labelings. This can f.e. be a date or time.

If working with large data sets (e.g. time series) with hundreds of lines it is
usually not desired to get each and every data point labeled on the x axis.
Also, the x axis grid will shop up on every single data point. Often it would be
desired to have the grid for every 10th or 100th data point only. The same
applies to the x axis ticks.

In principle this could work equal to the y axis where such settings are
available on the "scale" tab: main interval and minor interval (I'm not sure
about the correct names; I have the German version.)

With such settings for the x axis it would f.e. be possible to create a chart
for a multi-day time series of hourly values in which the x axis is just labeled
once for each day and the x axis grid consists of merely one line per day.