We are implementing a form to be able to edit chart definitions, rather than having to edit the raw json.
There is an "Edit with form" tab on the chart definition pages and it takes you to:
Special:ChartWizard/Data:ExampleChart.chart
We should also support creating new charts, when visiting Special:ChartWizard. It can have a form that allows user to enter then name of the chart and then it can redirect the user to the subpage Special:ChartWizard/Data:ExampleChart.chart.