fix(charts): set reasonable default y-axis title margin to prevent label overlap (#38389)

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

superset-frontend/plugins/plugin-chart-echarts/src/Timeseries/EchartsTimeseries.test.tsx

@@ -158,7 +158,7 @@ const defaultFormData: EchartsTimeseriesFormData & {
   xAxisTitle: '',
   xAxisTitleMargin: 0,
   yAxisTitle: '',
-  yAxisTitleMargin: 0,
+  yAxisTitleMargin: 15,
   yAxisTitlePosition: '',
   time_range: 'No filter',
   granularity: undefined,

superset-frontend/plugins/plugin-chart-echarts/src/Timeseries/constants.ts

@@ -46,7 +46,7 @@ export const DEFAULT_FORM_DATA: EchartsTimeseriesFormData = {
   xAxisTitle: '',
   xAxisTitleMargin: 0,
   yAxisTitle: '',
-  yAxisTitleMargin: 0,
+  yAxisTitleMargin: 15,
   yAxisTitlePosition: 'Top',
   // Now that the weird bug workaround is over, here's the rest...
   ...DEFAULT_SORT_SERIES_DATA,

superset-frontend/plugins/plugin-chart-echarts/src/constants.ts

@@ -104,7 +104,7 @@ export const DEFAULT_TITLE_FORM_DATA: TitleFormData = {
   xAxisTitle: '',
   xAxisTitleMargin: 0,
   yAxisTitle: '',
-  yAxisTitleMargin: 0,
+  yAxisTitleMargin: 15,
   yAxisTitlePosition: 'Top',
 };
 

superset-frontend/plugins/plugin-chart-echarts/test/MixedTimeseries/transformProps.test.ts

@@ -112,7 +112,7 @@ const formData: EchartsMixedTimeseriesFormData = {
   yAxisBounds: [undefined, undefined],
   yAxisBoundsSecondary: [undefined, undefined],
   yAxisTitle: '',
-  yAxisTitleMargin: 0,
+  yAxisTitleMargin: 15,
   yAxisTitlePosition: '',
   yAxisTitleSecondary: '',
   zoomable: false,