Resolved: the y axis doesn't shows actual value for above categories
except for the bottom category, the values on the y-axis aren't actual values for other categories so for actual values we subtract lower limit from the upper on y-axis to find the actual number of that category, am i right?
yes that's exactly right, if you'd like the individual value for a category you'd need to subtract the values of the categories, which are underneath it on the chart.
Alternatively, if you're mostly interested in the actual values of each category and their progression with time you can use a line chart to display the categories - that way you'll have the actual values for each category straight away. This is the topic of the next section in the course, where we also touch on the link between line and area charts.