My app uses Android Plot for a simple XY plot (lines).

Just updated to API 28 (targetSdkVersion and compileSdkVersion)

It does not build anymore, see the output:

Warning: com.androidplot.pie.PieRenderer: can't find referenced method 'int save(int)' in library class android.graphics.Canvas
Warning: com.androidplot.ui.LayoutManager: can't find referenced method 'int save(int)' in library class android.graphics.Canvas
Warning: com.androidplot.ui.SeriesRenderer: can't find referenced method 'int save(int)' in library class android.graphics.Canvas
Warning: com.androidplot.ui.widget.TextLabelWidget: can't find referenced method 'int save(int)' in library class android.graphics.Canvas
Warning: com.androidplot.xy.LineAndPointRenderer: can't find referenced method 'int save(int)' in library class android.graphics.Canvas
Warning: com.androidplot.xy.XYGraphWidget: can't find referenced method 'int save(int)' in library class android.graphics.Canvas

In fact Canvas.save(int) has been removed from the SDK in API 28. In was deprecated since 28, now it is fully removed and you should call Canvas.save() instead