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  Timestamp values not working for Python Chart
Posted by Bryan on Apr-12-2018 13:51
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test

  Re: Timestamp values not working for Python Chart
Posted by rapper man on Apr-12-2018 14:06
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HI there
I have attached my Python 3.6 script to generate a trend line chart. I have used the combo trendline.py and mysite example of django_viewportcontrol.zip from
http://www.chartdir.com/forum/download_thread.php?bn=chartdir_support&pattern=&thread=1204518493

I have verified the timeStamps which seems correctly created which are floats.
If I remove the line 'lineLayer.setXData(lst)", the chart gets generated correctly with dates.  When I add the line, there is a large red column in the chart which I attached. How do I correct this for the proper dates to be formatted along the X axis.

Here is a sample ChartTime column values of df which is the source for dates in the X axis.

              ChartTime
2017-12-02  6.364777e+10
2017-12-03  6.364786e+10
2017-12-04  6.364794e+10
2017-12-05  6.364803e+10
2017-12-06  6.364812e+10
2017-12-07  6.364820e+10
2017-12-08  6.364829e+10
2017-12-09  6.364837e+10
2017-12-10  6.364846e+10

Thanks
c2.png
trendtest.py
trendtest.py
1.92 Kb

  Re: Timestamp values not working for Python Chart
Posted by Peter Kwan on Apr-12-2018 15:24
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Hi Rapper,

The issue is because your code does not provide the x-coordinates for the trend layer. To solve the problem, please modify:

c2.addTrendLayer(closeData, 0x008000, "Trend Line").setLineWidth(2)

to:

c2.addTrendLayer2(lst, closeData, 0x008000, "Trend Line").setLineWidth(2)

If you do not provide the x-coordinates, the x-coordinates will be assumed to be the array index 0, 1, 2, 3 , ... So in your chart, the line layer will have very large x-coordinates like "6.364777e+10", while the trend layer will have small x-coordinates 0, 1, 2, 3, .... By default, the x-axis is scaled to include all the data. So the x-axis scale must be something like 0 to 70000000000, and all the points of your line layer will all be around x = 6.364e+10.

In brief, in ChartDirector, if no x-coordinate is provided, the x-coordinates are implicitly 0, 1, 2, 3.... If you set the x-axis labels using Axis.setLabels, the labels will be at x = 0, 1, 2, 3, ... too. So the label will match with the data points. Note that both the data points and the labels will be equally spaced in the x-direction, as their coordinates are equally spaced (0, 1, 2, 3, ...). The labels are only for human reading.

If x-coordinates are provided, ChartDirector will auto-scale the x-axis to include the data range (or you can use Axis.setDateScale or Axis.setLinearScale to specify the scale). The data points will be positioned using the x-coordinates. For example, if the x-coordinates are 10, 20, 30, 50, the first 3 points will be equally spaced in the x-direction, while the last point will have double the spacing.

In many financial charts, all data points are plotted as equally spaced in the x-direction, regardless of whether they are equally spaced in date/time. For example, non-trading days (Saturdays and Sundays, national holidays, non-trading days due to unpredictable natural phenomena such as hurricane or typhoon, etc) are skipped and will not result in a larger spacing between the data points. So it is more common to use x-labels instead of x-coordinates for financial charts.

Regards
Peter Kwan