When in stack chart mode, increase horizontally the call stacks hover zone so that it's easy to understand a profile without zooming

See this screencast:

https://user-images.githubusercontent.com/270485/118459792-5f311c80-b6fc-11eb-9dd4-7291362a9972.mov

it's really hard to understand what those narrow stacks are, one needs to zoom and it's fairly time consuming.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-388)