+Patrick Honner wrote a post titled:

'When Desmos Fails' which you should go read. In it he shows a quirk about

Desmos (a free online graphing calculator) that seems to not be able to correctly graph around the

*removable discontinuity* $(-2,-1)$ of the following function:

$$f(x)=\frac{x+2}{x^2+3x+2}$$

People who understand this better than me say it might have something to how javascript handles floats...

Anyway I thought I'd see how

+Sage Mathematical Software System could handle this. Here's a Sage cell with an interact that allows you to zoom in on the point (click on evaluate and it should run, it doesn't seem to fit too nice embedded in my blog so here's a link to a standalone Sage cell:

http://goo.gl/WtezZ4):

~~It looks like Sage doesn't have the same issues as Desmos does. This is probably not a fair comparison, and needed a bit more work than Desmos (which I can't say I've used a lot) to get running but I thought it was worth taking a look at :)~~

EDIT: IF you Zoom in more you do get the same behaviour as Desmos! I thought I had zoomed in to the same level as

+Patrick Honner did but perhaps I misjudged from his picture :)

Here's the same thing in Sage (when setting $z=10^7$ in the above code):