I don't think you can export a custom chart. Here are a few options that you have:
Pin the chart to a dashboard and share the dashboard with the persons that you want to view the chart. The dashboard can be viewed even without a Dynatrace login once they have the link.
Use the API to extract the metrics that you used in your chart. This does not create the chart for you it would just pull out the raw metrics which you could put in a table or spreadsheet for viewing.
While you won't be able to automate a report on a Custom Chart, you can in fact export the chart data to CSV, XLS, or an SVG image file: https://www.dynatrace.com/news/blog/create-ad-hoc-...