williams-et-al-1983-apple-ii-computer-graphics

128 APPLE II COMPUTER GRAPHICS71¥l PRINT "J F M A M J J A S 0 N D J F MA E A P A U U U E C 0 E A E AN B R R Y N L G P T V C N B R";72¥l VTAB (2¥l)This section labels the horizontal axis by a method which makes up insimplicity what it lacks in style. Lines 650-69(/) draw the little tic-mark onthe axis, and line 71(/) prints the labels in the four lines of text at the bottomof the Hi-Res page. Line 71 (/) was actually designed after the graph hadbeen plotted by using the "Escape Mode" editing features of the Apple. Ifyou use HGR2 or leave Hi-Res space below the graph, you could label thehorizontal axis in the same way we labeled the vertical.75¥l REM * PLOT DATA POINTS *78¥l HCOLOR = 779¥l HPLOT 24, 16¥l-DP(¥l) * SCL8¥l¥l FOR I = 1 TO NP%-181¥l X = 24 + I * SP%82¥l Y = DP (I) * SCL83¥l IF X > 279 THEN PRINT CHR$ (7) ;"CHART TOO WIDE" : GOTO 86¥l : REM (END )84¥l HPLOT TO X ,16¥l-Y85¥l NEXT IThe heart of the routine is in this short section. Line 79(/) plots the fi rst pointto establish an initial point for the line, then line 84(/) plots the rest of thepoints. The X-coordinate is the left side of the frame (24) plus the offset foreach point. DP(4) will be plotted at 24 + 4 * 18, or 96.The Y-coord inate is calculated by multiplying the data value by thescaling factor. It should seem curious that the Y-coordinate is not plotteddirectly, but is subtracted from 160. Remember that the Apple thinks (/) is atthe top of the screen and 16(/) at the bottom, so the quick little subtractionturns things around to put (/) on the bottom where you usually want it.The 15 data points are stored in DP((/)) through DP(14). This is why theFOR/NEXT loop established in line 8(/)(/) only goes to one less than thenumber of points (NP%-1).Functional PlottingMany times a graph may be drawn from a function or formula instead ofdata points. Listing 11-4 is a short example which graphs a couple offormu las. Try it out!1¥l REM FUNCTION DEMO2¥l REM3¥l DEF FN A(X) .¥l2 * x /\ 2 + 54¥l DEF FN B(X) . ¥l¥ll * x /\ 3