Hello. 1- If you are refering to the Pareto chart for the standarized effects in a design of experiment, the reference line is the quantile in the Student's t-distribution or the Lenth’s pseudo-standard error (PSE) . See e.g. http://support.minitab.com/en-us/minitab/17/topic-library/modeling-statistics/doe/factorial-design-plots/what-is-a-pareto-chart-of-effects/ 

For the Lenth’s Pseudo-Standard Error see http://www.tandfonline.com/doi/abs/10.1080/00401706.1989.10488595#.VsYoquYposI

2- If you mean the traditional Pareto chart the dotted line is the cumulative frequency. See the description here (http://support.minitab.com/en-us/minitab/17/topic-library/quality-tools/quality-tools/pareto-chart-basics/).

Regards

Hello. 1- If you are refering to the Pareto chart for the standarized effects in a design of experiment, the reference line is the quantile in the Student's t-distribution or the Lenth’s pseudo-standard error (PSE) . See e.g. http://support.minitab.com/en-us/minitab/17/topic-library/modeling-statistics/doe/factorial-design-plots/what-is-a-pareto-chart-of-effects/ 

For the Lenth’s Pseudo-Standard Error see http://www.tandfonline.com/doi/abs/10.1080/00401706.1989.10488595#.VsYoquYposI

2- If you mean the traditional Pareto chart the dotted line is the cumulative frequency. See the description here (http://support.minitab.com/en-us/minitab/17/topic-library/quality-tools/quality-tools/pareto-chart-basics/).

Regards

This answer is not helpful at all.

Abubakr. Whis is this not a helpful answer to you? All details are indeed given in the links?

and now in a language a beginning student will understand. if i understand this kind of chart i wouldn't search for the origin of that reference line