You can take advantage of these new graphical instructions (actually they were added over 1 year ago, in V11):

Xshift      : defines an abscissa coordinate in pixel offset from the anchor point
Yshift      : defines an ordinate coordinate in vertical pixel offset from the anchor point
Index       : Defines an abscissa coordinate in barindex (classic way)
Value       : Set an ordinate coordinate in price/value (classic way)
bottomright : bottom-right corner of the chart, the offset values will always be negative to shift left (X axis) and positive to shift up (Y axis).
bottomleft  : bottom-left  corner of the graph - the offset values will always be positive in order to shift to the right (X axis) and up (Y axis).
topright    : top-right    corner of the graph - the offset values will always be negative in order to shift to the left (X axis) and down (Y axis).
topleft     : top-left     corner of the graph - the offset values will always be positive in order to shift to the right (X axis) and negative in order to shift down (Y axis).
right       : middle-right point  of the graph - the shift values will always be negative to shift to the left (X axis) and can be positive or negative to shift up(+)/down(-) (Y axis)
left        : middle-left  point  of the graph - the shift values will always be positive to shift to the right (X axis) and can be positive or negative to shift up(+)/down(-) (Y axis)
top         : middle-top   point  of the graph - the shift values can be positive or negative for shift right(+)/left(-) (X axis) and will always be negative to shift down (Y axis)
bottom      : middle-low   point  of the graph - the shift values can be positive or negative for shift to the right(+)/left(-) (X-Axis) and will always be positive in order to shift up (Y-Axis)
middle      : middle       point  of the graph - the shift values can be positive or negative for shift to the right(+)/left(-) (X-Axis) and can be positive or negative for shift up(+)/down(-) (Y-Axis)

so that your code could be written as (see attached pic):

defparam DRAWONLASTBARONLY = true
////Drawtext CENTERED at barindex
//drawtext("asdadsfadfasdffasfasfd",BarIndex, 1  )
//drawtext("asdfa  sfd asdfaf" ,BarIndex, 2)
//drawtext("asd fasfd" ,BarIndex, 3)
//--------------------------------------------------------------------------------------------------------------------
drawrectangle(-370,0,-235,-20)                 anchor(TopRight) coloured("YellowGreen") bordercolor("Black")
drawtext ("asdadsfadfasdffasfasfd", -305 ,-10) anchor(TopRight) coloured("Black")
//
drawrectangle(-370,-20,-235,-40)               anchor(TopRight) coloured("Silver")      bordercolor("Black")
drawtext ("asdfa  sfd asdfaf",      -323 ,-30) anchor(TopRight) coloured("Black")
//
drawrectangle(-370,-40,-235,-60)               anchor(TopRight) coloured("PapayaWhip")  bordercolor("Black")
drawtext ("asd fasfd",              -341 ,-50) anchor(TopRight) coloured("Black")
//--------------------------------------------------------------------------------------------------------------------
RETURN 0, 4

x-2.jpg