fix(grapher): fix chart icon on first load for line charts

[sophiamersmann]

Fixes an issue where the chart icon changes unexpectedly.

This is what happens (see video below):

• On first load, the user lands on the map
• Since we're on the map, a single year is selected
• With a single year selected, the line chart turns into a discrete bar chart
• That's why we show the bar chart icon in the content switchers
• If the user clicks on the chart tab, the time is reset and a start and an end year is now selected
• The chart icon changes from a bar chart icon to a line chart icon (unexpected!)

We would rather show a line chart icon from the beginning.

Screen.Recording.2023-10-16.at.11.57.17.mov

Examples:

• Line chart by default: https://ourworldindata.org/grapher/co-emissions-per-capita
• Discrete bar chart by default (but actually a line chart): https://ourworldindata.org/grapher/fish-and-seafood-consumption-per-capita

[sophiamersmann]

@marcelgerber can you have a quick look at the code? Line charts turning into discrete bar charts make choosing the correct custom icon kind of complicated... I worry that the code I've written is difficult to understand :(

[marcelgerber]

Nice! Alternative PR up at #2777, where I explain why I did things differently, and could get rid of some of the confusing parts :)