There is apparently an issue in Vega which makes it so that Altair cannot display data pulled from columns which have a period in their name --- the world_countries.csv file in chapter 6 of httlads has a column called 'Pop. Density'. If one tries to use Altair to display a chart with data from that column, it will not be able to display it properly. For example, it occurs when one creates a scatterplot of 'Pop. Density' with 'GDP'.
There is a workaround by calling the column in Altair with 'Pop. Density' rather than 'Pop. Density'. However, this seems idiosyncratic enough that it would be better to rename the column in world_countries.csv to Pop Density in order to avoid this issue.
There is apparently an issue in Vega which makes it so that Altair cannot display data pulled from columns which have a period in their name --- the world_countries.csv file in chapter 6 of httlads has a column called 'Pop. Density'. If one tries to use Altair to display a chart with data from that column, it will not be able to display it properly. For example, it occurs when one creates a scatterplot of 'Pop. Density' with 'GDP'.
There is a workaround by calling the column in Altair with 'Pop. Density' rather than 'Pop. Density'. However, this seems idiosyncratic enough that it would be better to rename the column in world_countries.csv to Pop Density in order to avoid this issue.