sorry for my explanation. for the question
The question is just wether your hardware is fast enough to do that.
It is able to do as fast as much.

for( i=0; i<n; i++ )
{
QString result=read_data.at(i);
result.remove(0,10);
qDebug("%s",result.toLatin1().data());
y[i]=result.toLatin1().toFloat();
points << QPointF( i, y[i] );
// check wether curve is updated, if not do it here
plot->replot;
plot->show;
}
It is the part of your code. In this you write the code

plot->replot;
plot->show;

in for loop.
My question is, loop will execute for every i value till it reaches the count.

the code in the loop will excute for every "i" value so plot->show is also excute for every value.

means it will open the plot window for every i value.

I have tested your code, it will displaying so many no of windows

how to modify that code to plot the graph continuously.