I've had my Nexus 5 for about a year and a half now, but lately it's showing weird charging behaviour (check battery history graph):

What's going on and how can I fix it?

I've had my Nexus 5 for about a year and a half now, but lately it's showing weird charging behaviour (check battery history graph):

I leave it charging every night and it's supposed to reach 100%, now it doesn't even go beyond 48%. What's going on and how can I fix it?