Welcome to the world of IoT!
Yes, the maximum number of samples displayed by a graph - in real time- is limited to 50.
If you need to plot more than 50 samples, you can do it in offline (Export data in CSV file..).
By the way, May I ask why you need to plot more than 50 samples in the same graph ?
Based on our experience, users looking for ergonomic in real time visualizations plot less that 30 samples in one graph.