OFFICE-2119 date axis, here logarithmic scale

[Regina Henschel <regina.henschel@libreoffice.org>]

Hi members,

the attached documents contain charts to illustrate, why I think, we 
should not allow a logarithmic date axis.

Below the charts are screenshots of the axis settings in LibreOffice.
The file is "ODF 1.2 extended" with the LibreOffice extensions, 
otherwise you would not get the date axis in the line chart.

The sheet "Real" contains the line chart with date axis. I have chosen 
the unit day, because then you get the same distance of 1 as with the 
number interpretation of a date.

The next chart is an XY-chart with the same axis settings as in the line 
chart. From the included regression curve you can see, that LibreOffice 
indeed uses the internal number of a date for calculating.

The third chart is an XY-chart with logarithmic x-axis scaling. I have 
uses that, because a logarithmic scaling is not possible for a line 
chart in LibreOffice (and should not become possible). If you want to 
get some grid lines and labels, you need a very small "major interval". 
The chosen value 0.05 has no special meaning and are only used for to 
get some grid lines.

You will notice, that the curve and the grid in linear and logarithmic 
scale are nearly the same, so using logarithmic scale gives no further 
insights.

The sheet "Fake" contains two XY-charts, where I have shifted the null 
date by subtraction of 40179. The UI of LibreOffice has no option to set 
the null date to an arbitrary value. I have extended the x-values to 
cover the values in column A too and then used these high values for a 
secondary x-axis. The label of this secondary x-axis is shown as date 
values. The labels of the first x-axis are not shown, but the first 
x-axis is used for calculating the regression curve. Therefore you get 
here a different equation than on the sheet "Real".

Here too the "major interval" value in the logarithmic scale is a small 
decimal, otherwise you get not enough labels. So this shows, that 
shifting the null date would not solve the problem, that integer values 
for the "major interval" do not allow useful grids and labels.

Kind regards
Regina

Attachment: DateAxis.ods
Description: application/vnd.oasis.opendocument.spreadsheet