###################################################################
######### Create gray scale with lighter tone being smaller positive values
######### and darker tone being higher positive values
######### It is used by function survPMFPlot()
###################################################################
grayScale <- function(x, X0, X1, Y0, Y1, method, lowerThreshold = -Inf, YValueForLowerThreshold = NA){
### there are points (X0,Y0) and (X1, Y1) that we aim for
if(X0 == X1){
stop("X0 should not be equal to X1.")
}else{
if(!(method %in% c(1, 2, 3))){
stop("Argument 'method' can equal only to 1, 2, or 3.")
}
if(method == 1){
a = (Y0 - Y1)/(X0 - X1)
b = Y1 - a*X1
res = a*x + b
}
if(method == 2){
a = (Y1 - Y0)/((X1 - X0)^2)
b = -2*a*X0
c = Y0 + a*X0^2
res = a*x^2 + b*x + c
}
if(method == 3){
a = -(Y1 - Y0)/((X1 - X0)^2)
b = -2*a*X1
c = Y0 - a*X0^2 + 2*a*X1*X0
res = a*x^2 + b*x + c
}
}
res[x < lowerThreshold] = YValueForLowerThreshold
res
}
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