The interactive feature you will implement is a cross-hair that follows the X position of the mouse cursor and the Y position of the graph.
The cross-hair and event listeners
First create a second canvas-element that we will use to draw the cross-hair on. This canvas-element must be positioned over the graph and be the same size. By drawing all the interactive features on this second canvas you will not have to redraw the whole graph when you update the cross-hair. You can use a trick to erase the canvas, overwrite its width or height properties with their previous values.
To draw the cross-hair in the correct position you need the position of the mouse (relative to the canvas). This position can be determined by listening for mouse move events and implementing an appropriate callback. Register an event listener on the top canvas-element that listens for mousemove-events. In the callback access the clientX and clientY properties of the event that was fired, these properties have the mouse position relative to the whole document (not in “canvas” coordinates). Use getBoundingClientRect() of the canvas-element to correct the mouse position. As always you should use console.log to see what the actual coordinates are.
Now that you have access to the x-coordinate of the mouse relative to the canvas element you can find the height of the graph at that position. You do this by transforming the x-coordinate relative to the canvas back to “data” coordinates (use last week’s createTransform) and looking up the nearest temperature value. That temperature value can then be used to find the y-position relative to the canvas. You now have the coordinates where the horizontal and vertical lines of the cross-hair should cross. Draw the cross hair.
Extra credit: The tooltip and setTimeout
The additional interactive feature you can implement is a tooltip that appears several seconds after the mouse stopped moving and shows the actual, precise, data value under the cursor. The tooltip should be a div-element that you position absolutely on top of the two canvas-elements and slightly offset from the mouse pointer. First create this div make sure it shows the temperature and date of the point that is under the cross-hair. Make the tooltip move with the mouse (that is update its position when a mousemove-event is fired).
The tooltip should not be showing the entire time and thus we will implement a delay to the moment that it shows up. Make sure that the drawing and updating of the tooltip is encapsulated in a function. Use the setTimeout function to delay the drawing of the tooltip (so make it call your tooltip drawing function as a callback). Make sure you make the tooltip invisible when the mouse starts moving again.
Figure 1 shows an example (not complete) of the full graph.