The x value of the joystick controls how big of an arc that the sweeper covers.
The y value of the joystick controls the speed of the sweeper.
When I change the joystick position, the sweeper changes its tempo and movement correspondingly.

int pos;
int speed;
#include <Servo.h> 
#define SERVO_PIN 9

Servo servo;
//copied from servo sweeper sketch
void linear_move(int start, int end, float speed = 60.0)
{
 // Specify the number of milliseconds to wait between updates.
 const int interval = 20;

 // Compute the size of each step in degrees. Note the use of float to capture
 // fractional precision. The constant converts speed units from milliseconds
 // to seconds: deg/step = (deg/sec) * (sec/msec) * (msec/step)
 float step = speed * 0.001 * interval;
 
 // Declare a float variable to hold the current servo angle.
 float angle = start;

 // Begin a do-loop. This always executes the body at least once, and then
 // iterates if the while condition is met.
 do {
 servo.write(angle); // update the servo output
 delay(interval); // pause for the sampling interval

 if (end >= start) {
 angle += step; // movement in the positive direction
 if (angle > end) angle = end;
 } else {
 angle -= step; // movement in the negative direction
 if (angle < end) angle = end;
 }
 } while (angle != end);

 // Update the servo with the exact endpoint before returning.
 servo.write(end); 
}

void setup() {
 Serial.begin(9600);
 pos = 90;
 speed = 60;
 servo.attach(SERVO_PIN);
}

void loop() {
 int x = analogRead(A0);
 int y = analogRead(A1);
 Serial.print(x);
 Serial.print(":");
 Serial.print(y);
 Serial.print("\n");
 
 if (x < 300) {
 speed = 30;
 } else if (x > 700) {
 speed = 120;
 } else {
 }

 if (y < 300) {
 pos = 15;
 } else if (y > 700) {
 pos = 90;
 } else {
 }

 Serial.print(speed);
 Serial.print("(s):(p)");
 Serial.print(pos);
 Serial.print("\n");
 linear_move(90 - pos, 90 + pos, speed);
 linear_move(90 + pos, 90 - pos, speed);

}