A small update. Dealing with the past four years of glucose data I found another little trick needed when integrating with org-mode.
The date format in org-mode is
[2015-12-03 Thu] for inactive dates, and
<2015-12-03 Thu> for active dates. R doesn't know what to do with the extra characters. So an extra bit was needed:
data$Date <- as.Date( gsub( "^.", "", data$Date), "%Y-%m-%d") That strips off the leading character, and then the date extraction works. It ignores the extra stuff at the end.
Now I've got four years of glucose data scatter plotted. It needs some trend lines to eliminate the noise, but it's already made some things obvious.