Export charts in excel (.csv exists, .xls/xlsx needed)

This feature has been added in R3.0 via a feature flag.

Reference: https://preset.io/blog/superset-3-0-release-notes/#full-csv-export

How to export Full CSV?

When the ALLOW_FULL_CSV_EXPORT feature flag is enabled, dashboard users can click the overflow menu on any Table chart and export the full dataset as either a CSV or Excel export. A word of caution though: depending on the size of that dataset, this may cause a meltdown for your database and/or your browser. That's why it's behind a feature flag (and off by default).