add percent error bar to gl2d log error bar mock

Author: etpinard

test/image/baselines/gl2d_error_bars_log.png

@@ -1,10 +1,18 @@
 {
     "data": [{
+      "name": "array error bars",
       "type": "scattergl",
       "y": [10, 2e4, 5e6],
       "error_y": {"array": [5, 1e4, 4e6]}
+    }, {
+      "name": "50% percent error bars",
+      "type": "scattergl",
+      "x0": 0.2,
+      "y": [10, 2e4, 5e6],
+      "error_y": {"type": "percent", "value": 50}
     }],
     "layout": {
-      "yaxis": {"type": "log"}
+      "yaxis": {"type": "log"},
+      "showlegend": false
     }
 }