Allow reload of graph data upon a flot zoom action

This change is necessary to allow us to limit the flot data that we send over to the client. We want to filter the amount of data to a manageable size. This means, however, that if the user zooms, that we need to send over a new set of data.

The javascript change in this pull request handles that. To enable it, an extra css class must be set by the backend that claims to support it (see the changelog). This way it is backwards compatible.