If you are running anything up to and including A2016 retail, it will be a traditional chart.
A2019 retail or 365 has both and it depends which button was used to create the chart:
If its a traditional chart, Arnel's explanation is what you need:
However that doesn't work for modern charts which are easier to create but you have less control over the appearance. Go to the Format tab in the Chart Settings and see if you can find anything there
I don't use them as I find them less useful for my needs so am not an expert on these . However in A365 you can search the Help menu for this item 'Create a chart on a form or report'. Hopefully that will give you the answer you need. If so, please post back for the benefit of others