Stores data for a trend graph.

 In one DsTrendCurve object, up to 10 attributes can be specified.
 The attribute values will be stored with the same frequency, and the
 curves will be plotted in the same trend window.

 The names of the attributes is specified the the Attribute array.
 For every attribute, a storage area object also has to be specified.
 An attribute may be referred to by several different DsTrendCurve
 objects, for example to store at different frequencies.

 The sampling rate is specified the the ScanTime attribute.

 Storage buffers has to be supplied by the user, one buffer for the time axis data, and
 one for each configured attribute. Object of type CircBuffer has to be used, eg
 CircBuff1k, CircBuff2k, CircBuff10k, CircBuff20k, CircBuf100k and CircBuff200k.
 A CircBuff1k object will for example store 250 samples of size 4 bytes and a
 CircBuff200k 50000.

 The datatypes that can be stored is
 - pwr_tFloat32, pwr_tFloat64
 - pwr_tInt32, pwr_tUInt32, pwr_tInt16, pwr_tUInt16, pwr_tInt8, pwr_tUInt8

 The time is stored as one or two UInt32, dependent on the selected TimeResolution.
 The default time is of size 4 bytes where the seconds from epoch is stored. With time
 resolution nanoseconds the size is 8 bytes, and one word with the nanoseconds are added
 for each sample.

 DsTrendCurve objects is handled by the fast server rt_trend.
 The base frequency for the fast server process is configured in
 the DsTrendConf object.

 Trend data can be stored by a user defined server, for example by the plc program.
 In this case a number of setting has to be done:
 - Function is set to 1.
 - AttributeType should be set, eg pwrs:Type-$Float32.
 - Attribute should be set to an arbitrary attribute. The only function for the attribute
   is the curve name.
 - ScanTime should be set to the time difference between two samples.
 In the Buffer object, the Size (max number of samples) and ElementSize in bytes should be set.
 To fill in the samples the functions cbuf_AddSample() and cbuf_AddTimeSample() can be used.
 In this example the trend is filled in from a DataArithm where the data buffer is connected
 to Da1 and the time buffer to Da2.

   classdef Da1 CircBuff10k;
   classdef Da2 CircBuff20k;
   pwr_tTime time;

   time_GetTime(&time);
   cbuf_AddSample(Da1, &A1);
   cbuf_AddTimeSample(Da2, &time);

• CircBuff10k
• CircBuff10k
• CircBuff10k
• CircBuff10k
• CircBuff10k
• DsTrendConf

 Optional text.

 Title of the trend window.

 Bitmask for trend function.
 - UserStorage    1     Data inserted by user.

 Scantime in seconds. Multiple of the base scantime configured in DsTrendConf object.

 The attribute type only has to be set if UserStorage
 is used, where the type is set in the form pwrs:Type-$Float32.

 Stores measurants data types. It has to be of any of
 the types pwr_tBoolean, pwr_tFloat32, pwr_tFloat64,
 pwr_tInt8, pwr_tInt16, pwr_tInt32, pwr_tUInt8,
 pwr_tUInt16, or pwr_tUInt32.

 The complete name of analog sampled attributes, for
 example PROC1-TEMP1.ActualValue. The attributes have to
 be represented by local objects.

 Circular buffer objects that contains the data of the curves.
 Objects for the databuffers has to be configured by the user. The extent of the
 buffer attributes should equal or exceed the storeage size, i.e. the size of the
 sampled attribute's type multiplied by NoOfSample.

 Time buffer. A circular buffer object that contain the time axis data for the curves.
 The object for the time buffer has to be configured by the user. The extent of the
 buffer attributes should equal or exceed the storeage size for the time data.

 Resolution of time scale, seconds or nanoseconds.
 Seconds will require 4 bytes per sample, and nanoseconds 8 bytes per sample.

 Length, in seconds, of the storage. If the time
 exceeds what is possible to store in DataBuffer, the
 storage is stopped when DataBuffer has been filled.

 Length, in seconds, of the storage that is displayed
 in the trend window.

 Length, in seconds, of the storage that is displayed
 in the trend window.

 Time between updates of the curve display in seconds.

 Curve window layout.

 Number of samples.

 Specifies the first element in the data buffer with valid data.

 Specifies the last element in the data buffer with valid data.