Why are chemical changes considered "unseen"

Because they occur at an atomic level, changing the actual structure of the thing.
Hope it helps

There is no graph to look at on here, but i shall explain them to you. a line graph is just in its title it’s a graph with a line running through the dots to show an increase or decrease. a bar graph is used to show how much or how little something is so a comparison. a scatter plot is used to graph points to tell the data. and i don’t know what a histogram is.