Fix/inconsistent proptypes of discrete color legend (#1147)

* fix(DiscreteColorLegend): Fixed props validation

- Updated PropTypes of DiscreteColorLegend to be consistent with DiscreteColorLegendItem

* fix(DiscreteColorLegend): Updated documents

docs/legends.md

@@ -19,7 +19,7 @@ Currently following types of legends are supported:
 
 #### items (required)
 
-Type: `Array<string|{title: string, color: String, strokeDasharray: string, strokeStyle: string, strokeWidth: number, disabled: boolean}|react element>`
+Type: `Array<string|{title: string|react element, color: String, strokeDasharray: string, strokeStyle: string, strokeWidth: number, disabled: boolean}|react element>`
 
 Array of items that should be shown on the legend. The array should consist from either objects (`title`, optional `color`, optional `strokeDasharray`, optional `strokeStyle`, optional `strokeWidth`, and optional `disabled` flag) or strings (treated as titles). The stroke properties should match those in your series (see [line series](line-mark-series.md))
 

src/legends/discrete-color-legend.js

@@ -71,7 +71,7 @@ DiscreteColorLegend.propTypes = {
   items: PropTypes.arrayOf(
     PropTypes.oneOfType([
       PropTypes.shape({
-        title: PropTypes.string.isRequired,
+        title: PropTypes.oneOfType([PropTypes.string, PropTypes.element]).isRequired,
         color: PropTypes.string,
         disabled: PropTypes.bool
       }),