We Know Who You Are. We Know Who Your Talking About. We Just Don't Care All That Much.
First, note that some of the candidates are not included. We get only data on "major" candidates. So the information provided here about who is speaking the most is biased. But that presumes that we are uninterested in the agenda-setting funciton of some candidacies - Tom Tancredo, for instance, was hardly "major" and was mostly unmentioned, but he claims credit for getting immigration talked about in the primaries.
Second, note that the graphic focuses on the number of times a candidate mentions another candidate rather than substantive issues or topics. For that we have to go to this much less cool graphic (and even then we must assume that merely mentioning something is meaningful in one or another obvious way). So, in that sense we are getting only 'horserace' information. That may not be bad, but it is crucial to note that the cool graphic is not providing nformation about candidates and their positions. It is providing (at best) getting information about who thinks who may be politically challenging. (In other words, lack of mentions is also important in this context as a sign of political irrelevance.)
Third, given that it maps campaign interactions in primaries there is scant mention by either Repulicans or Democrats of those in the "other" party. The one primmary exception seems to be Hilary, who even the Republicans tend to mention. No surprise in any of this; it falls into the "Grandma knows that" category.
Finally, note that the person who is perhaps most mentioned (among Democrats, at least) is not included. That would be the dishonorable 'W' himself. Ooopps!