As others have suggested a SMU is the correct tool for this, they are the test instrument of choice for most kinds of static semiconductor testing. A lot of modern SMUs have built in graphing capability, older ones usually don't.
However you might find that a SMU capable of handling 100W can be rather expensive. So unless you plan to also use it for other things it might not be worth it. But since solar panels are always used in the regime of them producing power you can get away with just an electronic load.
Just find a electronic load with some sort of automation connectivity, RS232 USB GPIB LAN..etc. This is a good example for the use of instrument automation. The instrument is capable of doing the measurement at one data point, then you just use a PC with a script to run the instrument trough all the points and plot a graph. This can graph points as fast as the instrument can measure them. If this is something that is done regularly the same automation script can also generate a PDF report of the test, store the report somewhere, print it out etc. The report part is something standalone instruments don't do, but can sometimes take longer to make manually than even the measurement takes. Also since the script sets up the instrument for the test, it means that you eliminate operator errors, as it can easily happen that he puts in a wrong setting.
This PC automation part does require some programing skill tho since there are not much in the way of ready to go software tools while its dificult for a ready to go software tool to be flexible enough to cover all possible test setups with like >5 instruments working together.