I recently got a request to change a trend line on one of our dashboards and when looking at the docs - It says the following:
Trend lines Another useful option for line, area, bar, and scatter charts is trend lines. If you have a question where you’re grouping by a time field, open up the visualization settings and turn the
Show trend linetoggle on to display a trend line. Metabase will choose the best type of line to fit to the trend of your series. This will even work if you have multiple metrics selected in your summary. But it won’t work if you have any groupings beyond the one time field.
This leads me to believe Metabase is picking which type to be used, but does not say which? It would appear that all are using Linear, but would like to be able to pick Logarithmic.