An abstract conceptual overview graph of changes in hemline heights (skirt lengths) in middle- and upper-class western women's clothes over two hundred years (1805-2005).  The curve in this image is not based on exact numerical data, but instead presents a summary of broad general trends (i.e. the graph is more qualitative than quantitative, or "not to precise scale").
Before World War I, the hemlines of middle- and upper-class adult women's ordinary public clothes in western societies had varied only between floor length and an inch or two above ankle length (at most) for centuries.  This graph shows how things then changed in a previously-unprecedented way.

On the left, hemline height is indicated as being at ankle length (or slightly above) at the chronological start date of 1805 (i.e. 200 years before the date the first version of this chart was made), then dropping to floor-length ca. 1835, where it stayed for most of the remainder of the 19th century (with a few temporary excursions back to ankle-length -- see File:1794-1887-Fashion-overview-Alfred-Roller.GIF).  During WW1, hemlines rose far above ankle-length relatively quickly -- and then in the mid 1920's (after a brief dip) rose almost all the way to knee length.  During the period from the early 1930's to the mid 1960's, hemlines fluctuated in a zone which was quite different from the zone where hemlines had fluctuated during the 19th century.  In the late 1960's, hemlines rose significantly above knee length for the first time.  In the early 1970's, some women stayed with the miniskirt, some women went to the other extreme of ankle-length "granny dresses", while fashion designers tried to push an intermediate "midi" skirt length (see illustrations from September 1971 issue of Women's Wear Daily on p. 473 of Survey of Historic Costume ISBN 1-56367-142-5).  The strong rejection by women of the attempt to impose the "midi" as the new norm marked the end of only one skirt-length at a time -- while fashion trends continued to come and go, from the 1970's on it was no longer true that a woman had to wear one particular socially-predominating skirt-length or be considered almost hopelessly unstylish.  Instead, a variety of skirt-lengths now became acceptable (though after the early 1970's, the miniskirt itself didn't return as a mainstream fashion until the mid-1980's) -- and of course, in many contexts women are free to wear trousers instead of a skirt or dress.  This era of relative fashion freedom is shown as the dispersed grey area on the right of the chart.
The curve and gray area shown in the graph are loosely based on information from  a number of sources and historical illustrations.  The graph itself was made from scratch by H. Churchyard in April 2005, and modified for Wikipedia and uploaded to Wikimedia Commons in June 2008.
Graphic made by H. Churchyard in April 2005, modified for Wikipedia and uploaded to Wikimedia Commons June 2008.