Fix missing _legend mapping for facet model
Fix missing _legend mapping for facet model

cc: da39a3ee5e6b4b0d3255bfef95601890afd80709@domoritz

uwdata/vega-lite,uwdata/vega-lite,uwdata/vega-lite,vega/vega-lite,vega/vega-lite,uwdata/vega-lite,vega/vega-lite,uwdata/vega-lite,vega/vega-lite,vega/vega-lite