I have a collection of highly irregular sampled data. The gap between measurements can be few seconds, or few weeks or few decades ... What are the techniques to plot irregularly spaced time series data?
Just plot observations against timestamps and connect the dots.
For example, I measure my weight every time I get to the gym, which happens rather irregularly, indeed:
Depending on what information you want to show, you may also want to plot the length of each interval between successive observations, e.g.:
You could create a "time map" which is a type of scatterplot. Each event is a point, with the x-value being the time elapsed since the previous event, and the y-value is the waiting time until the next event. See this blog or on Twitter, check out #time_map_viz for examples.
You can color-code the dots to carry additional information. To deal with varying timescales, you can use logarithmic axes.