components / legend / add hmaxheightratio, hmaxheightratio attributes

  - Replace hardcoded horizontal legend max height ratio of 2 with
  user options (default is still 2).

Author: Nementon

src/components/legend/attributes.js

@@ -29,6 +29,24 @@ module.exports = {
         editType: 'legend',
         description: 'Sets the color of the border enclosing the legend.'
     },
+    hmaxheightratio: {
+        valType: 'number',
+        min: 2,
+        dflt: 2,
+        role: 'style',
+        editType: 'legend',
+        description: [
+	    'Sets the max height ratio (layout / ratio) of the visible legend when horizontaly aligned.',
+	    'Default value is 2; the legend will take up to 50% of the layout height before displaying a scrollbar',
+	].join(' ')
+    },
+    hmaxheight: {
+        valType: 'number',
+        min: 0,
+        role: 'style',
+        editType: 'legend',
+        description: 'Sets the max height (in px) of the horizontaly aligned legend.'
+    },
     borderwidth: {
         valType: 'number',
         min: 0,

src/components/legend/defaults.js

@@ -121,6 +121,8 @@ module.exports = function legendDefaults(layoutIn, layoutOut, fullData) {
     coerce('xanchor');
     coerce('y', defaultY);
     coerce('yanchor', defaultYAnchor);
+    coerce('hmaxheightratio');
+    coerce('hmaxheight');
     coerce('valign');
     Lib.noneOrAll(containerIn, containerOut, ['x', 'y']);
 

src/components/legend/draw.js

@@ -602,12 +602,18 @@ function computeLegendDimensions(gd, groups, traces, opts) {
     var isBelowPlotArea = opts.y < 0 || (opts.y === 0 && yanchor === 'top');
     var isAbovePlotArea = opts.y > 1 || (opts.y === 1 && yanchor === 'bottom');
 
-    // - if below/above plot area, give it the maximum potential margin-push value
+    // - if below/above plot area, give it the [user defined] maximum potential margin-push value
     // - otherwise, extend the height of the plot area
-    opts._maxHeight = Math.max(
-        (isBelowPlotArea || isAbovePlotArea) ? fullLayout.height / 2 : gs.h,
-        30
-    );
+    if (isBelowPlotArea || isAbovePlotArea) {
+      if (opts.hmaxheight !== undefined) {
+          opts._maxHeight = opts.hmaxheight;
+      } else { 
+          opts._maxHeight = fullLayout.height / opts.hmaxheightratio; 
+      }
+    }
+    else { 
+        opts._maxHeight = Math.max(gs.h, 30);
+    }
 
     var toggleRectWidth = 0;
     opts._width = 0;