Introduction ============ plotypus is a `Python `_ library and command line tool for modelling and plotting the `light curves `_ of periodic `intrinsic variable stars `_. Certain types of variable stars can be used as `standard candles `_, and are therefore essential in understanding the structure of the Universe. A property of `classical Cepheid variable stars `_ known as the `period-luminosity relation `_ was discovered in 1908 by `Henrietta Swan Leavitt `_. This relation depends on knowing the `period `_ of oscillation and the mean `magnitude `_ of a sample of Cepheids at a similar distance. In order to obtain these two parameters for a given star, its brightness must be observed over a length of time so that a `periodogram `_ may be used to find its period, and `regression `_ to fit a `periodic function `_ to the observations. plotypus provides both a command line interface and Python API for finding periods and fitting `Fourier series `_ to observations, and plotting the resulting light curves. A number of light curve parameters are obtained, including amplitudes :math:`A_i`, phase shifts :math:`\phi_i`, aplitude ratios :math:`R_{i1}`, and phase deltas :math:`\Phi_{i1}`. Finding periods is computationally intensive, and therefore existing periods can be provided ahead of time, either obtained previously with plotypus, or externally, from sources such as the `OGLE-III `_ catalog.