[Feature Request] Allow to set fixed boundaries for data parameters

[TheNewJavaman]

Maybe I missed this in the docs, but instead of automatically using the greatest/smallest data points as the dimensions, can you allow me to set the bottom as, say, 0.0 and the top as 100.0?

[TheNewJavaman]

Just by the way, I fixed my own use case by modifying the original flutter_sparkline.dart file and using in my own personal project. I just changed the bounds for _max = 100.0 and _min = 0.0