title: p5.js - change trail of shapes being drawn
body: <p>In my program, running your mouse over the screen will leave a trail of black, semi-transparent circles. I want to be able to change that by pressing the 'i' key, at which point it should draw a trailing blue line. Currently, upon pressing the 'i' key, it does draw a trailing blue line, but that line is surrounded by black, semi-transparent circles. If I try to call background, it solves the circle problem, but makes the line's trail disappear shortly after being drawn.</p>
  
  <pre><code>    if (mouseY &lt; 417) { //canvas is 500x500
        noStroke();
        fill(0,100);
        ellipse(mouseX,mouseY,20,20);
     } 
  
     if (keyIsPressed &amp;&amp; key == 'i') {
       if (mouseY &lt; 417) {
         stroke(0,0,255);
         line(mouseX,mouseY,pmouseX,pmouseY);
       }
     }
  </code></pre>
  
  <p>Please help!</p>