unified hoverlabel: do not process MathJax in hover since it's not supported

without this fix, in mock "ohlc_first", the library tries to render MathJax
and the result is broken

Author: antoinerg

src/components/legend/draw.js

@@ -494,6 +494,7 @@ function setupTraceToggle(g, gd) {
 }
 
 function textLayout(s, g, gd, opts) {
+    if(!opts._main) s.attr('data-notex', true); // do not process MathJax if not main
     svgTextUtils.convertToTspans(s, gd, function() {
         computeTextDimensions(g, gd, opts);
     });