In the Dashboard Builder, the horizontal axis labels on various dashboard visuals can be adjust to be shown vertically, horizontally, or at an angle.
The "Best Fit" option...
This option allows the labels in your visuals to be adjusted automatically as you resize or make other changes to them.
Here we see our column chart with the labels on the horizontal (X) axis shown horizontally:
If we were to decrease the width of the chart a little, the labels would start to get crowded. With the "Best Fit" setting in place, the Dashboard Builder will automatically adjust the labels. In this case, they are switched to show at an angle:
If we were to resize the chart by making it much narrower, the labels would again start to get crowded:
So, the Dashboard Builder will automatically adjust the labels so that they display vertically to keep them as readable as possible:
You can set the label rotation to any option you like and it will stay that way. The "Best Fit" option, allows the labels in your visuals to be adjusted automatically without you having to make any additional changes.