When creating a dashboard, the available charts change based on the context chosen.
By default, the context is Summary. With the Summary context setting, the available views display summary data for the current group context of the dashboard. The Summary setting does not require you to select a specific group or item. Summary views dynamically update the context when you change the context of the page.
If you explicitly choose a group the same views are available as the default context
If you choose a device/component, the views change pertinent to what is applicable to the device/component chosen.