The HTML reports give you a lot of possibilities and flexibility. You can generate and customise any type of report with the help of the SIL™ Template language . Basically, as its name suggests, you are building your report based on a html template and injecting your data using Simple Issue Language™ (SIL) syntax. 

Configuring SIL Reporting Gadget

First you need to add the SIL Reporting Gadget to your dashboard. To do that, click Add new gadget. In the Add new gadget dialog, click Load all gadgets. Search for the SIL Reporting Gadget and add it to your dashboard.

To see more instructions about adding a gadget to your dashboard see Atlassian documentation.

The configuration screen is only available to Jira Administrators and System Administrators.

To configure the gadget you have to select the sil script file and add the default values for the parameters used in the selected script.