CVI PlotY: How to center the X-axis at zero?

[Andrey_Dmitriev]

Dear colleagues,

I have a simple (maybe stupid) question about graphs in NI CVI.

My graph is “symmetrical” (it’s a simple PlotY graph where the X-axis is shifted by an offset), and I would prefer to have zero displayed in the middle.

Which attribute can I use to adjust this:

I tried adding a cursor at the zero position and played around with the attributes all morning, but that didn’t help.

Any idea how I can fix this and make it look perfect (in generic way, the range could be differ from run to run)?

Thank you in advance.

[RobertoBozzolo]

It appears as if your graph is in autoscale mode in the X-Axis. If this is true, then you could try setting it to manual mode, calculate the appropriate range and set it to the axis before plotting the values.

[RobertoBozzolo]

Looking at your first picture, I suppose the value of 2 in the middle depends on some rounding on actual values. You could probably fix it with this code:

// Calculate actual range
MaxMin1D (array, nElements, &maxVal, &maxIdx, &minVal, &minIdx);

// Round to the next group of ten
max = Max (ABS_VAL (minVal), maxVal);
max = (ceil (max / 10.0)) * 10.0;

// Apply the range to the graph X-axis
SetAxisScalingMode (panelHandle, PANEL_GRAPH, VAL_BOTTOM_XAXIS, VAL_MANUAL, -max, max);

Max and ABS_VAL macros are part of the Programmer's Toolbox, defined in toolbox.h.

[Andrey_Dmitriev]

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@RobertoBozzolo wrote:

Looking at your first picture, I suppose the value of 2 in the middle depends on some rounding on actual values. You could probably fix it with this code:

// Calculate actual range
MaxMin1D (array, nElements, &maxVal, &maxIdx, &minVal, &minIdx);

// Round to the next group of ten
max = Max (ABS_VAL (minVal), maxVal);
max = (ceil (max / 10.0)) * 10.0;

// Apply the range to the graph X-axis
SetAxisScalingMode (panelHandle, PANEL_GRAPH, VAL_BOTTOM_XAXIS, VAL_MANUAL, -max, max);

Max and ABS_VAL macros are part of the Programmer's Toolbox, defined in toolbox.h.

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Yes, I've already tried rounding to the next multiple of ten, and sometimes it works (though not always; I think I've seen the same behavior even with rounding). I'll check a few different scenarios.

[UliB · Accepted solution]

I asked google AI how to set the x markers 

void InitializeSymmetricGraph(int panel, int control, double n) {
    SetAxisScalingMode (panel, control, VAL_BOTTOM_XAXIS, VAL_MANUAL, -n, n);
    // 10 Intervals create 11 ticks, 6th tick is at 0.0
    SetCtrlAttribute (panel, control, ATTR_XDIVISIONS, 10); 
}

Disclaimer: I have no LabWindwos/CVI installation, so I can not verifiy if this will work or if these parameters are real. I had to ask AI if ATTR_XNUM_DIVISIONS is correct, which obviously is not.

At least there is a help entry for ATTR_XDIVISIONS: https://www.ni.com/docs/de-DE/bundle/labwindows-cvi/page/cvi/uiref/cviattrxdivisions.htm 

[Andrey_Dmitriev]

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@UliB wrote:

I asked google AI how to set the x markers 

void InitializeSymmetricGraph(int panel, int control, double n) {
    SetAxisScalingMode (panel, control, VAL_BOTTOM_XAXIS, VAL_MANUAL, -n, n);
    // 10 Intervals create 11 ticks, 6th tick is at 0.0
    SetCtrlAttribute (panel, control, ATTR_XDIVISIONS, 10); 
}

Disclaimer: I have no LabWindwos/CVI installation, so I can not verifiy if this will work or if these parameters are real. I had to ask AI if ATTR_XNUM_DIVISIONS is correct, which obviously is not.

At least there is a help entry for ATTR_XDIVISIONS: https://www.ni.com/docs/de-DE/bundle/labwindows-cvi/page/cvi/uiref/cviattrxdivisions.htm 

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Yes, thank you, that does the job. It is exactly the attribute I was looking for. I was sure something like it existed for the scale, but I overlooked the Divisions setting. Sometimes I can't see the forest for the trees: