changing the range of x axis constantly causes more and more memory allocation

To reproduce:

1. Build and run the attached code, it is a modified qmloscilloscope example
2. run pmap

Current behavior:

• the memory allocation grows constantly and after a minute it starts to affect the fps

Expected result:

• the memory consumption would stay on the same level like the qmloscilloscope examples

Note:

• if no labels are displayed it still grows
• if opengl is turned of it still grows
• valgrind shows no memory leaks