[ncl-talk] data match days before plot

[Debasish Hazra]

Thanks Dave. Able to bring both data in julian day format and common
starting time. But having issues with two unequal records, one from the
model and one from the station observations (several days in obs are
missing). Attached are two examples of output. And the missing days are
random among different stations. Not able to use regline_stats etc because
of unequal length. How to match these two to create the same length
considering the random nature of missing days in Station obs. Any
help would be appreciated.

Thanks
Debasish

On Sun, Aug 30, 2026 at 11:56 AM Dave Allured - NOAA Affiliate <
dave.allured at noaa.gov> wrote:

> Oops, I forgot that you are making a scatter plot.  This means that you
> need an X axis that is linear in time.  Therefore, you must use a numeric
> time line, not a decimal coded time line.  So use the method that I
> mentioned first, to convert both inputs to a common time line such as "days
> since".
>
>
> On Sat, Aug 29, 2026 at 10:53 AM Dave Allured - NOAA Affiliate <
> dave.allured at noaa.gov> wrote:
>
>> Put both the obs and model data on a common time line.  For the obs
>> (station) data, it looks like you need to combine the year number into the
>> calculation.  You did not show any time information for the model data, but
>> I am sure it is something simple like "days since" or "hours since".  I
>> suggest convert both to "days since", since the obs data is already based
>> on day numbers.
>>
>> Also when calculating the new time lines, you must pick a common starting
>> point.  I suggest YYYY January 1, where YYYY is the starting year of either
>> data set.
>>
>> Alternatively, you could use a decimal coded time line such as YYYYDDD or
>> YYYYMMDD, rather than a pure numeric time line.  With this type, dates are
>> absolute in one sense, so there is no need to pick a common starting date.
>>
>> For the scatter plot, you compare both time lines and pick out the dates
>> in common.  This is possible with any kind of time line, as long as both
>> are computed accurately, and roundoff error is resolved.  My final advice
>> is, pick whichever method seems easiest to create the time lines.
>>
>> NCL has date functions to assist with creating and converting time lines.
>> https://www.ncl.ucar.edu/Document/Functions/date.shtml
>>
>>
>> On Thu, Aug 27, 2026 at 2:20 PM Debasish Hazra via ncl-talk <
>> ncl-talk at mailman.ucar.edu> wrote:
>>
>>> Hi,
>>> I am trying to plot a scatter plot using Ncl and plotting daily averaged
>>> station data vs model output using more than 2 years data. However, I am
>>> seeing more than one data gaps in station data, like below (first value is
>>> in Julian days, second is precip value for a station of that day), but
>>> model data has values in all days.
>>>
>>> 350,0.418833
>>> 351,0.494576
>>> 355,0.389460
>>> 356,0.406334
>>> ...SNIP...
>>> 362,0.113083
>>> 363,0.124250
>>> 364,0.081895
>>> 365,0.159218
>>>   2,0.125749
>>>   3,0.217326
>>>   4,0.276310
>>>
>>> How to match those two data set in terms of  exact days before the
>>> scatter plot ? Any help ?
>>> Thanks
>>> Debasish
>>>
>>
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