I am about to design a web-page. In this process I need to draw diagrams that show
the statistic of a certain event. This diagram has to be updated at least every hour. Do
any of you know of program that already excists, which I can use?
Thanks

depends on how your website is served!
if you run apache on unix you can install the gd-libraries, and php.

with gd support you can write scripts in php that can create png and jpeg files. the good thing about php is that you can filter for example your server logfiles, and based on this data you can make your images created.
this also means that your data is always up to date because the moment someone watches the statistics on the apprpriate webpage the images are created on the fly using the latest data.

hope this is what you are looking for.
if so, the go over to the php forum and search for "gd library"