If you use Strava to estimate your average power then you should also use Veloviewer.
Veloviewer is a very cheap PC download that logs into your Strava account and extracts your ride data for analysis.
It will plot your average power against time on a graph so you can spot any upward (hopefully) trends (amongst a million other things you can waste your day plotting).
Here X = date, Y = average power for the ride and the size of circle is proportinal to the distance, large circle = large ride.
To me, if you squint slightly, and plot a rough trend, it's upward from October 2014 (where I started using Strava more)
So I drew my own (guesstimate) trend line and it equates to an increase from 136W to 159W (average power over rides of all distances, from 20km to 620km) from October 2014 to June 2015. The big circles to the bottom right are 400km and 600km Audaxes (power conservation mode)
Ideally you'd use it to analyse the same ride over many rides. I'm sure you can plot that but I just got very bored.