How can I create up to 80 (but in principle an N number of) short vertical lines in a such way that their color would represent a light spectrum ?
I am sure this can be done in a simple straightforward way. I tried to experiment with setting a rectangle filled with a grid of colors as a background and starting from this, but I stuck. The final effect would look like a discrete spectrum of light. Something like this: http://www.astronomynotes.com/light/discretbc.gif
Any help is appreciated.
Thanks,
Jakub

Do you really need many objects with one color? You can use one large rectangle and create a linear gradient with as many color stops as you need to create a spectrum.

If you really need the many objects solution use the Tiled clones command under Edit. You will need to create a thin rectangle without a stroke and Unset it's fill (it will appear black). Then go to Edit > Clones > Create tiled clones.
Use 1 row and as many columns you need (you'll be able to remove the clones, change settings and try again later if you don't get what you need right away). Symmetry - simple translation, leave everything as it is except the Color tab; change Initial color to red, and change Hue per column to 2%.

You can maybe also achieve it by using Extensions > Generate from path > Interpolate, create lines for major colors in the appropriate position, and choose the With Style option.

Thanks prkos ! That's exactly what I needed (the Tiled clones solution). Good stuff.
I am very impressed with Inkscape. I've been using it for 3 days and already managed to achieve quite a lot.
Jakub