charts: add some comments; refactorings
charts: add some comments; refactorings

yagebu/fava,beancount/fava,aumayr/beancount-web,beancount/fava,aumayr/beancount-web,yagebu/fava,beancount/fava,beancount/fava,yagebu/fava,beancount/fava,aumayr/beancount-web,yagebu/fava,aumayr/beancount-web,yagebu/fava