Handle non-R-compatible column names

[KarinSchork]

E.g. if column names in the original files contain special characters (like brackets or µ) or if column names start with a number (e.g. 0h, 24h).

These columns names do not fit to the rules for valid column names in R.
Then, R will replace the special characters with a dot and write and X before numbers using the make.names()-Function.

The question is, should we handle this somehow? Because it looks ugly in plots. Or should we insist that the column names of the input table are already valid column names?