Actual Time Spent vs Estimated Effort comparison report

A visual representation of work items (User Stories, Bugs and Tasks) organized by the percentage by which the actual Time spent on a work item exceeds the Effort assigned to it.

Let's create a new Visual Report for this purpose (Create > Report > Chart). We need to select Bugs, Tasks and User Stories as 'Entities' for our report, specify the Projects and Teams needed in the 'Projects and Teams' selector and add the following filter to the 'Filter' area:
?TimeSpent > Effort and Effort > 0

After the report is created, let's drag and drop the 'Name' parameter to X-axis, 'Assigned User' to Y-axis and remove 'Count of records' calculation from axes at all.

Now we need to create a new calculation by hitting 'Add field or formula' button ('Formula' tab). Let's give a name to our formula, ex. 'Overtime ratio', and populate the formula area with the following one:

TimeSpent / Effort

After saving the newly created formula, let's drag and drop it to the 'Size' panel. Now you are able to easily recognize the work items with the bigger number of the overtime ratio:

After you set everything up, you may want to tune your chart a bit to show, lets say, only the work items with overtime ratio greater than 1.3. In order to do that, please click on 'Chart data' button > 'Add filter' and select the values you need from the drop-down lists as in the following example:

You may also want to drag 'Type' parameter to 'Color' panel to have your data grouped and colored by Entity Types (Bugs, Tasks and User Stories):

After you hit the 'Finish setup' button, you are welcome to drill into details of your report to find and check the work items which required much more time to be implemented than it was estimated initially.