Relates to plotly#3735, adds itemsymbol: 'constant' legend option, which uses a circle symbol for all legend entries.

Author: brian.kotek

src/components/legend/attributes.js

@@ -77,6 +77,16 @@ module.exports = {
             'or remain *constant* independent of the symbol size on the graph.'
         ].join(' ')
     },
+    itemsymbol: {
+        valType: 'enumerated',
+        values: ['trace', 'constant'],
+        dflt: 'trace',
+        editType: 'legend',
+        description: [
+            'Determines if the legend items symbols use the symbol of the first point in each *trace*',
+            'or use a *constant* circle symbol.'
+        ].join(' ')
+    },
     itemwidth: {
         valType: 'number',
         min: 30,

src/components/legend/defaults.js

@@ -105,6 +105,7 @@ module.exports = function legendDefaults(layoutIn, layoutOut, fullData) {
     if(helpers.isGrouped(layoutOut.legend)) coerce('tracegroupgap');
 
     coerce('itemsizing');
+    coerce('itemsymbol');
     coerce('itemwidth');
 
     coerce('itemclick');

src/components/legend/style.js

@@ -25,6 +25,7 @@ module.exports = function style(s, gd, legend) {
     var fullLayout = gd._fullLayout;
     if(!legend) legend = fullLayout.legend;
     var constantItemSizing = legend.itemsizing === 'constant';
+    var constantItemSymbol = legend.itemsymbol === 'constant';
     var itemWidth = legend.itemwidth;
     var centerPos = (itemWidth + constants.itemGap * 2) / 2;
     var centerTransform = strTranslate(centerPos, 0);
@@ -203,6 +204,10 @@ module.exports = function style(s, gd, legend) {
             if(constantItemSizing && valToBound && cst !== undefined) {
                 valToBound = cst;
             }
+            
+            if(constantItemSymbol && attrIn === 'marker.symbol') {
+                valToBound = 'circle';
+            }
 
             if(bounds) {
                 if(valToBound < bounds[0]) return bounds[0];