first cut polar.usepolygons

- this adds polygons grid (and angular ax line) support to categorical
  angular axes
- had to add 'polygon mode' to pathSector() and isPtWithinSector()
- polygon vertices match the angular ticks
- snaps radialaxis.angle to vertex angles
- needed special care for sector and on angular drag

Author: etpinard

Diff for: src/plots/polar/layout_attributes.js

@@ -122,11 +122,6 @@ var radialAxisAttrs = {
     // span: {},
     // hole: 1
 
-    // maybe should add a boolean to enable square grid lines
-    // and square axis lines
-    // (most common in radar-like charts)
-    // e.g. squareline/squaregrid or showline/showgrid: 'square' (on-top of true)
-
     editType: 'calc'
 };
 
@@ -272,6 +267,19 @@ module.exports = {
     radialaxis: radialAxisAttrs,
     angularaxis: angularAxisAttrs,
 
+    usepolygons: {
+        valType: 'boolean',
+        role: 'style',
+        editType: 'plot',
+        dflt: false,
+        description: [
+            '...',
+            'Has an effect only when the angular axis has `type` *category*.',
+            'Note that `radialaxis.angle` is snapped to the angle of the closest',
+            'vertex (so that radial axis scale is the same as the data scale).'
+        ].join(' ')
+    },
+
     // TODO maybe?
     // annotations:
 

Diff for: src/plots/polar/layout_defaults.js

@@ -177,6 +177,10 @@ function handleDefaults(contIn, contOut, coerce, opts) {
 
         axOut._input = axIn;
     }
+
+    if(contOut.angularaxis.type === 'category') {
+        coerce('usepolygons');
+    }
 }
 
 function handleAxisTypeDefaults(axIn, axOut, coerce, subplotData, dataAttr) {