I’d think it would be impossible to make sense of your 27-series chart by using different colors. How about this approach, which shows all the data, but only emphasizes one series at a time:

Easier Interactive Multiple Line Chart

I recently created a line chart with 27 different series, i.e. all European Union’s Member States.
With the standard color palette (and also with the one I got from the Excel dashboards as advertised below :-) ), it is really a nightmare because all colors seem identical or the contrast is weak (yellow on white chart background).
You could ask : “Why do you want such a mess of lines ?”
And I would reply: “Just because God wants it, whoever God is )

Hence it would be good idea to develop a range of color palettes, hopefully by Excel version.
I am sure that Excel geeks, like myself, would be even ready to buy them, :-)

Good idea. It isn’t perfect, I only changed the colors needed for the graded bar fills, plus I lightened up the grays. Anyway, here it is:

Bar_Chart_Graded_Colors.xls

I concluded the same about the readability of the overlapping bar chart in The Problem with Marimekkos. I’m finishing up the series with Marimekko Replacement – 2 by 2 Panel.

For one, when you look at the chart, the blank space (between the dark blue and lighter shades of blue) can confuse as it gives an impression of area chart rather than bar chart. I liked your chart version : http://peltiertech.com/images/2009-02/MariBar_PanelColumns.png compared to what you have shown here. I guess it is much more easier to read.