javascript - Drawing rotated text on a HTML5 canvas

Question

Part of a web application I'm developing requires me to create bar graphs to display various information. I figured, if the user's browser is capable, I would draw them using the HTML5 canvas element. I have no problem drawing lines and bars for my graphs, but when it comes to labeling the axes, the bars, or the lines I ran into a snag. How do I draw rotated text onto a canvas element so that it lines up with the item it is labeling? A couple examples include:

• Rotate text 90 degrees counter clockwise to label the y-axis
• Rotate text 90 degrees counter clockwise to label bars on a vertical bar graph
• Rotate text an arbitrary amount to label lines on a line graph

Any pointers would be appreciated.

Answer 1

Posting this in an effort to help others with similar problems. I solved this issue with a five step approach -- save the context, translate the context, rotate the context, draw the text, then restore the context to its saved state.

I think of translations and transforms to the context as manipulating the coordinate grid overlaid on the canvas. By default the origin (0,0) starts in the upper left hand corner of the canvas. X increases from left to right, Y increases from top to bottom. If you make an "L" w/ your index finger and thumb on your left hand and hold it out in front of you with your thumb down, your thumb would point in the direction of increasing Y and your index finger would point in the direction of increasing X. I know it's elementary, but I find it helpful when thinking about translations and rotations. Here's why:

When you translate the context, you move the origin of the coordinate grid to a new location on the canvas. When you rotate the context, think of rotating the "L" you made with your left hand in a clockwise direction the amount indicated by the angle you specify in radians about the origin. When you strokeText or fillText, specify your coordinates in relation to the newly aligned axes. To orient your text so it's readable from bottom to top, you would translate to a position below where you want to start your labels, rotate by -90 degrees and fill or strokeText, offsetting each label along the rotated x axis. Something like this should work:

 context.save();
 context.translate(newx, newy);
 context.rotate(-Math.PI/2);
 context.textAlign = "center";
 context.fillText("Your Label Here", labelXposition, 0);
 context.restore();

.restore() resets the context back to the state it had when you called .save() -- handy for returning things back to "normal".

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