Small backend issues related to plots

[annavik]

Fixed some plot feedback from Lars in #441. There are some small things left which requires backend changes:

• Detections per hour chart is a bit strange
• Restrict number of species if many in top species chart
• Hover label is sometimes a bit repetitive, for example in top species chart

For the hover label, I have made it possible to specify this from backend since the label might vary depending on what chart we are dealing with. Just pass a hovertemplate string: https://plotly.com/javascript/hover-text-and-formatting/#hovertemplate