* Antecipated creation of plots to before loading data.
* Moved most visual roles to plot-level visual roles. Some visual roles remain as chart-level visual roles: dataPart, multiChart.
* Visual roles from plots other than the main plot must be configured in that plot's visualRoles map.
* For runtime use, visual roles of every plot are made available at the chart-level visualRoles map and visualRoleList, under the id and name of the plot. The main plot's visual roles are also registered with no plot prefix and with the special "main" prefix.