Why don´t you try modulating your random signal with a sin(t)
function:
long n = 0;
double randomWeight = 0.5;
while(true) {
nextVal = range.getRandomValue();
double temp = AMPLITUDE*(randomWeight*((double)nextVal)+(1.0-randomWeight)*Math.sin(2.0*Math.PI*((double)n)*WIDTH_FACTOR));
nextVal = (long)temp;
n++;
System.out.println(nextVal);
StdDraw.point(prevVal, nextVal);
StdDraw.line(t-1, prevVal, t, nextVal);
StdDraw.show(100);
prevVal = nextVal;
t = (t+1) % 100;
if(t == 0){
StdDraw.clear();
}
The idea is to generate a sinusoidal wave function and add some random noise to it. You may want to add this noise to a square wave instead. I don´t really know very much about how an electrocardigram looks like but I suppose it must be governed by the heartbeat.
EDIT:
I have just checked how these signals look like:
It seems to me, a more accurate model would be given by a relatively flat random signal interrupted by periodic spikes:
long n = 0;
while(true) {
nextVal = range.getRandomValue();
if(n % SPIKE_PERIOD == 0) nextVal = SPIKE_APLITUDE*nextVal;
n++;
System.out.println(nextVal);
StdDraw.point(prevVal, nextVal);
StdDraw.line(t-1, prevVal, t, nextVal);
StdDraw.show(100);
prevVal = nextVal;
t = (t+1) % 100;
if(t == 0){
StdDraw.clear();
}