My statistics students (Bachelor Business studies; adult education) appreciate:

Wong (2010). Guide to Information Graphics. W.W. Norton.

It is a very basic text but includes lots of practical examples how to use graphs.

Hello Aswani Eedara,

Generally speaking, most graphs contain two trends, or  sometimes there will be two graphs with a trend in each. As you have rightly observed, the proper language for describing trends in line graphs or polygons requires a specific language tool kit.The following links can hopefully provide you with necessary guidelines and proper examples for describing trends in line graphs.

Best of luck,

R. Biria

https://bees4work.pbworks.com/f/Company+Trends+Student.pdf

https://lenagroeger.s3.amazonaws.com/papers/BarsLines.pdf

https://www.eslflow.com/describinggraphstables.html

Thank you Reza, very useful for us non-native speakers.

The last link does not work, though.

Thank you, Professors Leopold hamminger and Reza Bira. 

http://www.eslflow.com 

some more useful resources I found today.

Online resource:

https://www.ilc.cuhk.edu.hk/files/ChartsGraphs_Gilhooly.pdf

Book: 

Let's speak Business English a guidebook for the non-native speaker of English by Linda Cypres

https://books.google.co.in/books/about/Let_s_speak_Business_English.html?id=Zm_C0KGnKB4C

web.efzg.hr/dok/PSJ/vkabalin/graphs-web.ppt

https://emedia.rmit.edu.au/learninglab/content/resultsdiscussion

https://dspace.ut.ee/bitstream/handle/10062/18053/adjectives_and_adverbs_of_movement.html