displayName -> title, fixes #1238 (#1295)
displayName -> title, fixes #1238 (#1295)

* displayName -> title, fixes #1238

* Update fielddef.ts

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