We suggest using measure Open issues on Columns for the report and Issue type dimension with selection Bug on Pages. This measure will show you open bugs over time.
You can check out a report from Samples folder Sample created vs resolved issues we create in any account by default if you are importing sample reports. Here is this sample report in our demo account. The report uses measures Issues created and Issues resolved to represent incoming and outcoming issues, Open issues at any period, and the average time for resolution.
Please make sure you are not using filters on Pages by Status or Resolution that removed issues already resolved.
Daina / firstname.lastname@example.org