With just these 2 dates you can do a lot! A lot of this came from looking over what was presented by Mattia Battiston at a meetup not so long ago. In particular, I was interested in his observation that there is very little correlation between story sizes and leadtime. This is something I have battled before so I was interested when my own data showed a very similar pattern:
Looking at this you could infer that a 2 point story might take as long as n 8 point story, which is hardly useful. What I missed for a while is the upper bounds across all the story sizes. We have a couple of anomalies at the top but the vast majority are within 10 days - which you should know is the length of our sprints.
That caught my interest. Although we cannot say with any sort of accuracy how long a 5 point story will take - in this example the longest was 26 days and the lowest was 4 days - we can say that 95% of 5 point stories can be delivered within 10 days or less. You can repeat a similar statement for story sizes 8 and below.
So maybe we should revise what we are expecting from our estimation by the team. What we are very good at doing is working out what can deliver within our sprint window. What we actually want to know is how many stories we can expect to be delivered if we want to use this to forecast, which is our throughput.
For me, looking at leadtimes influenced how we were working on and preparing stories. Littles Law describes how we should see a lower leadtime if we lower our WIP. We think more about how more of us can work on a single story, this often starts in prep. We open less and this lowers WIP, decreasing leadtimes.
Having a leadtime which only just fits in a sprint window means we have limited options on how to plan the work in the sprint - we have to start everything on day 1. This would mean loosing fluidity - everyone would be one a single story from beginning to end. Not many options for us at all.
Lowering leadtimes by a few days gives us some flexibility in when we can start stories. We use our story points to influence when we start on things. Larger stories might need to go first but smaller stories run in parallel so that we can deliver throughout the sprint rather than everything at the last minute, which would probably create a bottleneck with testing.
For me the sizes of stories begin to matter less and less. They serve their purpose during prep and planning and help us get to the answer: "Will this fit in our sprint?". When we see a 13 we look to break it down further since we can see it is rarely delivered in less than 10 days, which does not give us enough options.
If we bring something in mid-sprint, we can look at the previous leadtimes and give a percentage "75% of previous 5 point stories have been completed in the time we have left" allowing us to make a commitment with some accuracy on success.
There are many more interesting things you can gleen from leadtimes and some basic spreadsheet fun, Mattia's presentation is a great place to start.