If I use Tk::Canvas and create boxes or dots for each aquired data point, soon I'll fill all the memory

How about using a line item, and use the itemconfigure method to add new points to the line? That way you would only use one entity and get a line graph of the temp against time.

GD was also tried, but we need more functionality for the generated graphics.

What beside plotting points did you need for your graph that GD didn't provide?

If GD really cannot provide what you need for legends, axis and stuff (though I cannot see why it wouldn't), the you might consider positioning the Tk::Photo with just the points plotted over the top of a tk::Canvas on which you draws the axis, legends etc.