For species with high observation counts, it is fine to bin by day. Doing so may reveal interesting features, such as spikes in the above data during the last week of April. As the bin width increases by row, W = 1,5,10,15,20,30, small-scale variations are lost in smoothing. Is there scientific rationale for excessively smoothing data?
My suggestion would be to make bin width a user-controlled parameter, and / or allow the graph to adapt to sample density, plotting by week or by day depending on sample size. I also voted for the previous request:
But think that allowing binning by week is only a start to what can be done in terms of improvement.