Creating a Sound Wave using P5.js

Question

Using P5.js, I'm trying to create a visual that gets drawn as an mp3 plays. As the song progresses, a rectangle is drawn on the canvas to illustrate its amplitude. I'm having a problem with spacing each rectangle. With the code I've written out, they get drawn right next to each other, but I ideally I would like 1 or 2px in between.

Here is what I have now:

This is what I'd like:

Any suggestions would be greatly appreciated! Here is my code:

var song
var button
var amp
var volHistory = []

function preload(){
    song = loadSound("next-to-me.mp3")
}

function setup(){
    createButtons()
    amp = new p5.Amplitude()
}

function togglePlay(){
    if(!song.isPlaying()){
        song.play()
    } else {
        song.pause()
    }
}

//draw is constantly being run
function draw(){
    //styling
    createCanvas(400, 150)
    background(245)
    stroke(0, 109, 203)

    //populate volHistory
    if(song.isPlaying()){
        var vol = amp.getLevel()
        volHistory.push(vol)
    }

    //iterate through volHistory and draw
    beginShape()
    for(var i = 0; i < volHistory.length; i++){
        var y = map(volHistory[i], 0, 1, height/2, true)
        fill(0, 109, 203)
        rect(i, y, 2, y, 25) //(x, y, w, h, radius)
    }
    endShape()

    //moves wavelength 1 index at a time
    if(volHistory.length > width - 10){
        volHistory.splice(0, 1)
    }

    //draw vertical line
    stroke(250, 30, 100)
    line(volHistory.length, 0, volHistory.length, height)
}

function loaded(){
    createButtons()
}

function createButtons(){
    button = createButton("<img style='width: 50px' src='http://www.stickpng.com/assets/images/580b57fcd9996e24bc43c4f9.png'/>")
    button.mousePressed(togglePlay)
    button.position(162, 50)
    button.style("background-color", color(0,0,0,0))
    button.style("border", 0)
}

Answer 1

To put space between the amplitude bars you can add an offset to each bar's x position. To make the bars vary in height according to the amplitude you can set each rectangle's height to the mapped amplitude and then center it by calculating its y position.

With the offset your draw function will look like this:

function draw(){
    background(245)
    stroke(0, 109, 203)

    //populate volHistory
    if(song.isPlaying()){
        var vol = amp.getLevel()
        volHistory.push(vol)
    }

    //iterate through volHistory and draw
  fill(0, 109, 203)
  var barWidth = 2;
  var offsetWidth = 5;
  var offset = 5;
    for(var i = 0; i < volHistory.length; i++){
        var barHeight = map(volHistory[i], 0, 1, 1, height)
        rect(i + offset, (height/2.0) - (barHeight/2.0), barWidth, barHeight);
        offset += offsetWidth;
    }

    //moves wavelength 1 index at a time and account for bar width and offset width
    if(volHistory.length * (offsetWidth + barWidth) > width - 10){
        volHistory.splice(0, 1)
    }

    //draw vertical line
    stroke(250, 30, 100)
    line(volHistory.length + offset, 0, volHistory.length + offset, height)
}

Note that in this draw createCanvas has been moved to setup