Otherwise, you could submit an enhancement request on bugzilla. With regard to other tools - there are a LOT of SIP based analysis tools most of them cost money. It's really meant more as a way to graph packet rates/counts over time. For example, I used tcpdump -XX to capture some artibrary packets and chose a short tcp one from the ascii dump: 16:51:27. But I don't think it will do what you want. If you are only interested in the timestamp of the packets, then you can snapshot a single packet in hex and replicate it, changing only the timestamp, using text2pcap, which is usually in the same package as wireshark. You could try playing with that to see if you can get what you want. There is also the 'Statistics->IO Graph', which is very customizable.

For example, clicking on the menu item 'Telephony->Voip Calls' brings up a dialog showing the various calls in the capture if you select one of those calls and click the 'Flow' button, it will show you another dialog with the timestamps on the left, and message flow in the mdidle (what is typically called a 'ladder diagram' in the SIP world).

There are ways of seeing various stats for SIP-based calls.

As far as I know, the exact graph you're looking for doesn't exist natively in Wireshark, at least in the sense that there's no such graph already created for you. Well I can take a shot, but I'm not sure I'm right.

Graph loss from tcpdump wireshark