Remember, this is not absolute battery usage, but relative percentages of what has been used. Marshmallow includes "doze", which idles most of the phone's background activity when it's sitting still untouched. I'll bet the long flat part of your slope, from about 9AM to 11AM, was when your phone sat still while you weren't using it. The end result is longer battery life when not in use, but WiFi accounts for a larger percentage of the lowered use while dozing.
When you are connected to a WiFi network, keep WiFi on always, even while sleeping. Otherwise it'll rely on LTE in the background and use more energy and more data.
On my Nexus 5, battery life since Marshmallow has gone from maybe 16 to 20 hours to more like 24 to 30 hours. During that same time, WiFi's percent of usage has risen from about 4-5% up to 25%.