PTPlot is a tool for exploring the gravitational wave power spectrum from first-order phase transitions, and evaluating the likelihood of detecting a signal with the LISA mission. The results plotted by this tool are based on the paper Detecting gravitational waves from cosmological phase transitions with LISA: an update, arXiv:1910.13125, which has been published in JCAP. Please cite both that paper, and this Zenodo deposit, if possible. The source for PTPlot is available at https://github.com/CFT-HY/PTPlot (previously https://bitbucket.org/dweir/ptplot). This version is a snapshot of the v1.1.0 release. Notable changes include: Additional documentation, including a technical note authored by Jenni Häkkinen describing the physics of PTPlot. This technical note is also directly included with this Zenodo deposit. Bug fixes to the plotting code. Old multithreading code has been removed. The LISA Cosmology Working Group watermark has been removed.