As with most visualizations, there are usually parameters that can be changed to alter its behavior. These parameters could affect anything from the way the visualization is rendered on the screen to how it processes the data it receives.
The developer of the custom visualization decides which options they want to expose to the user, unlike the built-in dashboards that come with Splunk. To access the options, put the dashboard in Edit mode by clicking the Edit button on the dashboard and then click on the paintbrush icon of the visualization. From there, you can go through the different options and update as necessary. When you're finished, you can click the X in the upper-right corner of the menu:
