Hi @cavour.martinelli, welcome on board.
first of all: before trying to plot the data received by an Arduino board (and a HC05 shiled), is everything fine with the BT comm's between app and Arduino ?
In other words, are the data correctly received by the app (i.e do you show them in a label, and are they as you expect ?).
Once you're done with that, you could have a look here:
As a general hint, please remember to make one step at a time: for example split your project in two, one part relevant to the BT comm's, and the second relevant to the plot of data (that, for simulation, you can inject from a list into the plotting procedure). When both parts work well, you can glue them together 
If you need further details, do no ìt hesitate to ask more.
Cheers.