Unless you see reported "Crawl Errors" or "Blocked URLs" then it would seem that missing graph data is simply "where the data is not available". However, Google does not appear to be particularly clear as to why these "gaps" occur, simply stating that it can take time for data to appear for new sites and to check the stats for both the www and bare domains (and setting a preference). Your site is not necessarily in error.
I had previously stated that these gaps were likely to be dates when Googlebot simply didn't crawl your pages and therefore has no data. However, I have since seen numerous occasions where a 0 (zero) value is in fact recorded on the graph, so that would not seem to be the case.
I would state, however, that I have only seen these gaps in the graph on new sites and on sites that have few pages and/or low traffic. Sites that receive a moderate amount of traffic (and hence have a higher crawl rate) don't appear to get these gaps in the graph, in my experience.
I have also found that the graphs don't necessarily correlate directly to my server stats. But in this case the figures are low (so as to be measurable) so there is margin for error.
Your site is still relatively new. To see your crawl rate increase, regularly add more pages, submit a sitemap, promote your site on the net to get more backlinks etc.