This new graph plots orbital period versus planetary radius for planetary candidates discovered by the Kepler Space Telescope (see also The Cosmos, Section 9.2d, pp. 240-243, and Figure 9-9, p. 239). The colored symbols represent how many planets orbit a given star, and the legend lists how many of this type of system are in the catalog. (For example, the catalog lists 2967 stars with only one planet orbiting them.) Planets with shorter orbital periods are overrepresented because geometric factors and frequent transits make them easier for Kepler to detect. The upward slope in the lower envelope of these points is caused by the difficulty in detecting small planets with long orbital periods, for which transits are shallow and few are observed.
Link: article in EOS; for more detailed information, see Lissauer, J. J., R. I. Dawson, and S. Tremaine (2014), Advances in exoplanet science from Kepler, Nature, 513, 336–344 (arXiv version available here).