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I'm trying to make a scatter chart of events, where I have day of week on one axis and time of day on another axis and the data plotted as points for each day+time event.

What I'm trying to get is a time+day distribution of "when things happened", but there's nothing to plot about what or how much happened.

Ideally I would also plot by month or year on the third axis, e.g. by colour, but that's just a nice-to-have.

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Answer to question below ... second attempt as my first was lost :-(

I'm using the ls --time-style=+"..." option, so have good control of the data. Here's an image of data and a basic scatter chart. Attemps to restructure the chart end up blank. There are 40k lines of data and trying to handle all also appears(!) to hang.

To do this, you need a scatter chart (XY chart). The X axis needs to be handled as numerical values, otherwise it will treat them as categories, with the values being just labels and each successive value stacked. Here's an example, and I'll explain how I did it:

The day values are numbers. You can get them by applying the WEEKDAY function to your dates (1=Sun through 7=Sat). These need to be stored as numbers, but if you want them readable as days, format them with a custom date format of ddd, which I did for the last seven values.

Each year is a separate data series, so you want the data presorted, which it looks like you have. Start the chart by selecting the first year's data (Day, time, year), which will give you the first series. For the series name, select one cell in the year column. Verify that the X axis range points to the day column range and the Y axis range points to the time column.

For each additional year, add a series with the + button at the bottom and repeat the process to specify the data ranges.

The automatic settings for the X axis will start at 0 and end at 8, which is fine. That will keep your data in the interior of the chart. It looks like LO Calc won't format the numbers for the X axis in a scatter chart. What I did was add an X axis title. For the title text, I entered the day names with spaces between them, then adjusted the space so the names lined up with the axis ticks. Then I formatted the axis to not show the axis labels.

This is how to get the chart to appear as you ask, but the 40,000 data points is a different issue. Assuming your computer has the resources to handle it, it will take some time. Your computer may be busy and locked up for minutes even with enough RAM. If it doesn't have a ton on RAM, the process could get extremely lengthy, swapping chunks to disk, even to the point of swapping "forever". You may need to divide the data into subgroups and plot those separately, or plot just a sample of the data (random sample or every Nth point), or a combination of methods. Even 10% of the data (4,000 points) is a lot of data for one graph, but you can experiment to see how much data your system can deal with.