Existing charts can be added to dashboards using the Add Report button in the top-right area of the screen. The same chart can be added to multiple dashboards, meaning if the chart is edited, all dashboards with this chart will reflect the change.
Note that using the Add Report button is not the same as using the Save As function in the chart editor. The Add Report button merely adds the chart to the dashboard whereas the Save As function creates a new version of an existing chart.
Here's an example: