do not use "on" because it is untranslatable.

Author: wxiaoguang

web_src/js/features/heatmap.js

@@ -23,7 +23,7 @@ export function initHeatmap() {
       heatMapLocale: {
         months: new Array(12).fill().map((_, idx) => translateMonth(idx)),
         days: new Array(7).fill().map((_, idx) => translateDay(idx)),
-        on: 'on',
+        on: ' - ', // no correct locale support for it, because in many languages the sentence is not "something on someday"
         more: el.getAttribute('data-locale-more'),
         less: el.getAttribute('data-locale-less'),
       },