I'm happy to say that since the last release, line chart and area chart also support this. You'll have to set
strictFirstColumnType: true in the options. This is still not documented (we have a lot of documentation backlog to go over). Currently the default value of
strictFirstColumnType is false, but in a few months it'll change to true, and later on it'll be completely removed so 'true' is the only option. X axis variable spacing will be triggered whenever you have a first column of type other than 'string'.