QSemaphore for 2 QThreads to Control 2 Real-time Plots

[Pandako]

Hello, I had a very confusing issue. I wrote a QT program using 2 threads.

• Main thread: GUI thread, and draw 2 real-time plots using 1 QCustomPlot widget. Their x axis are synchronized.
• Data thread: continuely fetch real-time data and process data. The raw data and the processed data would be drawn on 2 different real-time plots. I used a buffer to store data. After recieving some number of data points, it would emit functions to plot.

Here is the problem: The data thread can emit drawing action to 2 different plots in the main thread independently. I used QSemaphore to control the drawing part.
Global variables:

QSemaphore freeBytes(1);
QSemaphore usedBytes;

In data thread:

freeBytes.acquire();
emit updateChart1(data);
usedBytes.release();

In main thread:

usedBytes.acquire();
_chart->updateAndDraw1(data);
freeBytes.release();

This works pretty well if I only draw 1 plot, and I could draw real-time plot.
However, I don't know I could draw 2 plots. Let's say we have two functions updateChart1(data) and updateChart2(data) to update 2 plots recpectively. These two functions can be emitted at any time independently. They don't always plot at the same time, though updateChart1(data) and updateChart2(data) could be running at the same time. If I used QSemaphore, the thread would be blocked, and the second plot would be drawn with latency.
How would I draw this 2 real-time plots in this situation? Am I using QSemaphore wrong? Or are there any better ways to achieve this goal?
If there's anything unclear in my description, please tell me. Any help is highly appreciated!