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GRAPHING THE DATA

by George Martin

Start ý How I Did It ý Background Painting ý Data Plotting ý Sources and PDF

BACKGROUND PAINTING

I sized the picture box so that it fit comfortably on my screen. Iýve got a 17ý monitor and Iým using 1152 ý 864 (pixels) resolution. PictureBox is 12000 ý 7789 (twips). If youýve got a smaller screen, you could reduce the size of the PictureBox or keep the size large and implement scroll bars.

The pixel (short for picture element) is the smallest graphical unit of measurement on the screen. Pixel size and spacing is screen dependent (i.e., its dimensions vary with the system display and resolution). A twip, on the other hand, is a screen-independent unit of measure equal to one-twentieth of a printerýs point. There are approximately 1440 twips to an inch.

Next, I created a box that outlined the active area for the chart. The boxýs corner coordinates were:

TL_X 400 top left X

TL_Y 200 top left Y

BR_X 11200 bottom right X

BR_Y 7200 bottom right Y

And, I drew that box using the following VB statement:

oPrinter.Line (TL_X, TL_Y)-(BR_X, BR_Y), , B

Notice that I created the constants TL_X, TL_Y, etc. to define the borderýs outline. That way I could change the size by altering these constants and not rewriting the code.

I next put grid marks in the active area. I created STEP_Y and STEP_X constants for grid spacing with STEP_X being just an arbitrary setting equal to 500 twips, while STEP_Y was defined to represent 0 to 100% grid spacing on the y axis.

STEP_Y = (SIZE_Y / 255) * 25.5

Remember that we have channels 1ý4 that are 8-bit A/D inputs, and channels 5 and 6 that are 12-bit A/D inputs. The 8-bit inputs range from 0 to 255 counts, while the 12-bit inputs range from 0 to 4095 counts. So, a percentage seems like a good choice for y-axis scaling. You could choose engineering units or a nonlinear scaling that suits your specific application. Keep in mind that the more math you perform, the slower the graphing becomes.

The actual grid marks are drawn with this code:

Const GRID = 20

For x = TL_X To BR_X Step STEP_X

For y = BR_Y To TL_Y Step -STEP_Y

oPrinter.Line (x - GRID, y)-(x + GRID, y)

oPrinter.Line (x, y - GRID)-(x, y + GRID)

Next y

Next x

Grid markers are made by drawing a horizontal line then a vertical one. The width of the lines are defined by the constant GRID. If you change the value of GRID, the size of the hash marks change.

The y-axis scaling is printed using this routine:

px = TL_X ý 350 ' Start the text to the left of the box

x = 0 ' Value

For y = BR_Y To TL_Y Step -STEP_Y

oPrinter.CurrentX = px

oPrinter.CurrentY = y - 100

oPrinter.Print x

x = x + 10

Next y

The variable px is the x-coordinate for the label and it remains constant. The variable x is the numeric value for the grid marker. The variable y represents the y coordinate for the label, and it changes for each grid mark. As you can see, this code fragment steps through the grid axis, labeling each point.

x-axis labeling is similar, except that no scaling is implied. Larger numbers positioned on the right work best. The data is plotted from left to right until the end is reached. At that point, the process starts over again without erasing any of the previously plotted data. This plotting style is only useful for some monitoring situations, but you may wish to modify this charting technique to find one more useful for your application. Hopefully, Iýve given you enough of the basics to accomplish that.

py = BR_Y + 100 ' Start the text to the bottom of the box

y = 0 ' Value

For x = TL_X To BR_X Step STEP_X

oPrinter.CurrentX = x - 100

oPrinter.CurrentY = py

oPrinter.Print y

y = y + 10

Next x

I started to tune this loop to represent readings. If you change the size of any of the constants being plotted, how fast the readings come in, or the step size of the x direction, then all the scaling needs to be changed. I could do this with formulas, but that slows down the code. So, in keeping with the lean, mean design approach, this is it.

My last task is to print a list of the channels and the colors used for each channel. I filled in the boxes with the color just for clarity, using simple colors (the first six I came across) for the channels.