Plotting bar graphs using QwtPlotHistogram

[penny]

I need to plot bar graphs, such that not all of them begin from the axis.
If i use QwtPlotHistogram, i can plot all data but starting from the axis.
I have attached an image showing how i want my plot to look.
Any suggestions?

[mohini]

try overlapping the existing plot with another plot (disable its color)
Not sure if its a correct method But I got similar plot by doing this....

[Uwe]

i can plot all data but starting from the axis.

What type of data do yo have:

- [y1,y2] = f( x );
- [y1,y2] = f( [x1, x2] )

Or do you simply have an ordered array of [y1,y2] intervals that are unrelated to any x value ( beside of their position in the array ) ?

Uwe

[penny]

I have just an ordered array [y1,y2], unrelated to any x value

[Uwe]

Then better implement your own type of plot item.

Derive from QwtPlotSeriesItem and use samples like this:

class YourSample
{
public:
    YourSample();
    YourSample( const QwtInterval &, const QwtInterval & );
 
    bool operator==( const YourSample & ) const;
    bool operator!=( const YourSample & ) const;
 
    //! Interval
    QwtInterval interval[2];
};

class YourSample
{
public:
    YourSample();
    YourSample( const QwtInterval &, const QwtInterval & );

    bool operator==( const YourSample & ) const;
    bool operator!=( const YourSample & ) const;

    //! Interval
    QwtInterval interval[2];
};

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Then your series data object can return [ index - 0.5 , index + 0.5 [ as one interval and the other interval from your array.

class YourSeriesData: public QwtSeriesData<YourSample>
{
public:
    YourSeriesData( const QVector<QwtInterval> &intervals):
        m_intervals( intervals )
    {
    }
 
    virtual size_t size() const
    { 
        return m_intervals.size();
    }
 
    virtual YourSample sample( size_t i ) const
    {        
        QwtInterval interval( i - 0.5, i + 0.5, QwtInterval:: ExcludeMaximum);
        return YourSample( interval, m_intervals[i] );
    }
 
    virtual QRectF boundingRect() const
    {
        double xMin = -0.5;
        double xMax = size() + 0.5;
 
        double yMin = ...;
        double yMax = ...;
 
        return QRectF( xMin , yMin, xMax - xMin, yMax - yMin );
    }
};

class YourSeriesData: public QwtSeriesData<YourSample>
{
public:
    YourSeriesData( const QVector<QwtInterval> &intervals):
        m_intervals( intervals )
    {
    }

    virtual size_t size() const
    { 
        return m_intervals.size();
    }

    virtual YourSample sample( size_t i ) const
    {        
        QwtInterval interval( i - 0.5, i + 0.5, QwtInterval:: ExcludeMaximum);
        return YourSample( interval, m_intervals[i] );
    }

    virtual QRectF boundingRect() const
    {
        double xMin = -0.5;
        double xMax = size() + 0.5;

        double yMin = ...;
        double yMax = ...;

        return QRectF( xMin , yMin, xMax - xMin, yMax - yMin );
    }
};

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Of course the code above is for vertical bars ( v.v. to your screenshot ).

Implementing the virtual methods for your plot item should be straight forward ( or copy and adopt your code from QwtPlotHistogram ).

Uwe