To find gravity assists, you should look at the underlying information of a pork chop plot. Specifically, you want to look at the heliocentric velocities needed to start the assist, and the velocity gains from the assist. Keep in mind that you get extract the most momentum from the planet by doing a close fly-by.
People rarely generate and look at the pork chop plots in order to find good gravity assists. This is because they're relatively hard to read, and the scale will limit the precision the human eye can detect. Instead, mission designers set parameters of broad missions, and use trajectory finders. The most used one is undoubtedly NASA EMTG.
However, if you are looking for a tool readily available, I would recommend using this tool, designer, I wrote last year which found a novel impulse-burn only trajectory to Neptune. I admit that it's a bit of a pain to set-up: it'll probably take 30 or 45 minutes to download the NASA SPICE kernels, install the Go programming language, and compile the tool. But then, you configure the tool with a simple configuration file (such as this one for Neptune) where you can specify the precision of each flyby, the maximum $c_3$ at Earth departure, the resonance orbits, the maximum delta-V you want to apply during the gravity assist (that's called a powered flyby), and the broad spans of time you would like the vehicle to flyby each planet.