How to zoom into Mandelbrot set by clicking - processing

Question

I managed to create the Mandelbrot set in processing but am struggling to implement a zoom. Here is my code:
float minvalX = -1.5;
float minvalY = -1.5;
float maxvalX = 1.5;
float maxvalY = 1.5;
float angle = 0;
float di,dj;
int xPixel,yPixel;
void setup(){
size(500,500);
pixelDensity(1);
colorMode(HSB, 360);
}
void draw() {
scale(zoom);
float maxLoops = 100;
loadPixels();
float equationOneOriginal;
float equationTwoOriginal;
for (xPixel = 0; xPixel < width ; xPixel++) {
for (yPixel = 0; yPixel < height ; yPixel++) {
float a = map(xPixel+di, 0,width, minvalX, maxvalX);
float b = map(yPixel+dj, 0,height, minvalY, maxvalY);
equationOneOriginal = a;
equationTwoOriginal = b;
float n = 1;
while (n < maxLoops) {
float equationOne = a*a - b*b; //First part of the equation
float equationTwo = 2 * a * b; //Second part of the equation
a = equationOne + equationOneOriginal;
b = equationTwo + equationTwoOriginal;
if (abs(a+b) > 16) {
break;
}
n++;
}
if (n == maxLoops) {
pixels[xPixel+yPixel*width] = color(0);
}
else {
pixels[xPixel+yPixel*width] = color(n-(int)(n/360)*n, 360, (int)map(n*6, 1, maxLoops, 0, 360));
}
}
}
updatePixels();
}
void mousePressed()
{
if (mouseButton == LEFT) {
di = di + mouseX - int(width/2);
dj = dj + mouseY - int(height/2);
minvalX += 0.1;
maxvalX -= 0.1;
minvalY += 0.1;
maxvalY -= 0.1;
}
}
It zooms into where my mouse is but eventually it goes towards the middle and gets stuck there. I know it is to do with minvalX, maxvalX, minvalY and maxvalY but I don't know what to do to these values to get it to always zoom towards my mouse.