There are all sorts of intricate ways you can do this by changing the width
of images, etc...

However, such things always look unbelievably amateur, just like the
hyperlink that you provided...

Do yourself a huge favour and get hold of a 3rd-party charting add-in for
ASP.NET - there are literally hundreds of these, some of which are free, and
you'll be up and running in minutes rather than days.

I use this: http://www.advsofteng.com/cdnet.html

Not that hard to do, really. You don't need any images, just HTML elements
with CSS. The "background" would be a straightforward HTML table. You would
use a fixed pixel size for the width. For the bars in the foreground, you
would use absolutely positioned divs, with colored backgrounds, and ZIndex >
0. The sizes of these divs would be whatever height you want to assign them
(in pixels) and the width determined by the length of the time period
specified, relative to the width of the chart. The locations of them would
similarly be determined by the starting point relative to the width of the
chart, and the distance down from the top of the chart for each succcessive
div.

It would not be difficult to make the entire thing a Web Server Control that
generates the HTML and CSS dynamically, for use with any web application.