The legends on your chart can be customized in different ways. For instance, you can resize a legend icon, adjust its position on the chart, reverse legend order, add interactivity, and so on.

A detailed description on configuring legends, refer to the articles Legends and Adding Interactivity to Legends

However, to further customize your charts you can create custom legends by adding Annotations. Annotations are user-defined objects or shapes that can be created or positioned anywhere on a chart.

Refer to the KB article, Annotations for more information. 

The documentation article, Annotations also has a detailed description of annotations, its uses, and capabilities.