Changes in version 2.0.2
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Date: 2018-07-25
* add several arguments to `trendline()` function
* enable to draw confidence interval for regression models
* enable to choose which parameter (equation, R-square and p-value) to show
* enable to specify the character of R-square and P-vlaue (i.e. R^2 or r^2; P or p)
* add 'ePos.x' and 'ePos.y' arguments to specify the x and y co-ordinates of equation's position
* deleted the 'ePos' argument 


Changes in version 1.2.0
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Date: 2018-07-13
* change the expression for `model` of `exp3P` using a supscript
* add `trendline_summary()` function
* add `SSexp2P()` function
* add `SSpower2P` function
* add `Pvalue.corrected` argument in `trendline()` and `trendline_summary()` , for P-vlaue calculation for non-linear regression.
* add `Details` in `trendline()` and `trendline_summary()` 
* add `...` argument in `trendline()` as same as those in `plot()`

