First attempt - done!

C64/gr_basics.asm

@@ -16,8 +16,7 @@ unPlotAfterX
     lda oldplotH,x
     sta oldplot+1
 
-    lda oldply,x
-    tay
+	ldy #0
     lda oldora,x
     sta (oldplot),y
 
@@ -31,30 +30,24 @@ CheckX
     jcs EndOfUnPlot
 MakeUnPlot
     ; let's count coordinates taken from xdraw and ydraw
-    ;xbyte = xbyte/8
-    lda xdraw+1
-    lsr
     lda xdraw
-    ror ;just one bit over 256. Max screenwidth = 512!!!
-    lsr
-    lsr
-;---
-    tay
-    ldx WhichUnPlot
-	tya
-    sta oldply,x
-
+	and #%11111000
+	;sta xbyte
+    ;---
     ldx ydraw
-    lda linetableL,x
-    sta xbyte
-    sta oldplot
+	clc
+    adc linetableL,x
+	sta xbyte
+	sta oldplot
     lda linetableH,x
+    adc xdraw+1
     sta xbyte+1
-    sta oldplot+1
+	sta oldplot+1
 
     lda xdraw
     and #$7
     tax
+	ldy #0
 
     lda color
     bne ClearUnPlot
@@ -91,9 +84,6 @@ LetsCheckOverlapping
     lda oldplotH,x
     cmp oldplotH,y
     bne NotTheSamePlot
-    lda oldply,x
-    cmp oldply,y
-    bne NotTheSamePlot
     ; the pixel is in the same byte so let's take correct contents
     lda oldora,x
     sta oldora,y

scorchC64.asm

@@ -35,7 +35,7 @@
     icl 'definitions.asm'
 ;---------------------------------------------------
 
-FirstZpageVariable = $30 ; $57
+FirstZpageVariable = $58 ; $57
     .zpvar DliColorBack		.byte = FirstZpageVariable
 	.zpvar GradientNr		.byte
 	.zpvar GradientColors	.word