One of commonly used tools for plotting in Python is Matplotlib package. You need to install it (It is already included in Anaconda distribution (free) from Continuum Analytics )
Can you provide sample *.dat file to be plotted. *.dat is just a file extension, it doesn't tell anything about content of the file.
The basic plot package in Python is Matplotlib. In principle, you can plot whatever you want using this package. As you might want to plot a sequence of data related to time series, then Pandas is a useful tool. The Series and DataFrame in Pandas is extremely useful for time-series plotting.
As suggested by Mr. Kislov and Mr. Jing, you need Matplotlib package. But if you want to plot data related to time series then Pandas can be more useful.