Accepted answer

I've based this example on

Given that the data you have is already in JSON this example needs some changes. See for my version.

Changes are mostly around manipulating the data. To do this you want to nest the data so that values (date and casecount) sit under each groupname

processeddata = d3.nest()
    .key(function (d) {
        return d.groupName;

After that it's mostly a case of changing data accessor names. For more info on nest I always find this page helpful:

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