fix issue rendering one-day at edges of data domain after navigating from two-week view
calculate BG scale based on smbg and cbg if present so can render either or both
Merge pull request #11 from tidepool-org/jebeck/integrate-tideline-bugfixes

Fixes:

- issue rendering data when navigating back to one-day view from a two-week smbg at the very edges of the data domain
- potential issue rendering smbg data if no cbg data present