Should I explain the type of a graph in my Masters thesis? I've conducted a survey for my masters thesis and would like to show the results as a box plot:



(The x-axis lists different skill / familarity levels, ranging from "none" to "expert")

Should I mention that this graph is a box plot (in the caption or the surrounding text) so that people unfamiliar with it can look up how to interpret this type of diagram, or is it unnecessary information and I can assume everybody will understand the type of graph just by looking at it?

Context is a computer science masters thesis in German at a German university.
You should use your caption to explain what the graph is supposed to be indicating. i.e. "Boxplot of survey data showing generally higher level of experience with SVN than competing technologies". This achieves both, and further supports the text of your thesis, rather than explaining to the reader something that should be completely obvious (e.g. "This is a boxplot of the survey data"). If you include the type of chart in the caption then it will help if someone is unfamiliar with that type of chart and won't take up much more room than typing "Chart of survey data showing..."

That said (and slightly off-topic), a boxplot doesn't seem to be the correct graph to use to convey this information. A boxplot should be used for continuous data. Here you have discrete (categorical/ordinal) data. Perhaps a barchart (with the different categories shown in different colours might work better)? I'm struggling to understand what the "lower quartile" between "Keine" and "Grundkenntnisse" means for google docs. Is that 25% of respondents had Keine experience of google docs? Very confusing! It seems like you've decided you want to use a boxplot and are trying to find a way to use one.

Let's say I want to know how many people have expert understanding of SVN, how do I determine that from your graph?

However with a bar chart... it's clear that (for example) 5 people who responded were experts